PSAT Math : Arithmetic

Study concepts, example questions & explanations for PSAT Math

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Example Questions

Example Question #1996 : Psat Mathematics

If the price of a TV was decreased from $3,000 to $1,800, by what percent was the price decreased?

Possible Answers:

50%

20%

40%

60%

30%

Correct answer:

40%

Explanation:

The price was lowered by $1,200 which is 40% of $3,000.

Example Question #1 : How To Find The Percent Of Decrease

If a rectangle's length decreases by fifteen percent, and its width decreases by twenty percent, then by what percent does the rectangle's area decrease?

Possible Answers:

45

32

36

35

40

Correct answer:

32

Explanation:

Let's call the original length and width of the rectangle l_{1} and w_{1} , respectively.

The initial area, A_{1}, of the rectangle is equal to the product of the length and the width. We can represent this with the following equation:

A_{1}=l_{1}\cdot w_{1}

Next, let l_{2} and w_{2} represent the length and width, respectively, after they have been decreased.  The final area will be equal to A_{2}, which will be equal to the product of the final length and width.

A_{2}=l_{2}\cdot w_{2}

We are asked to find the change in the area, which essentially means we want to compare A_{1} and A_{2}. In order to do this, we will need to find an expression for A_{2} in terms of  l_{1} and w_{1} . We can rewrite l_{2} and w_{2} in terms of l_{1} and w_{1}.

First, we are told that the length is decreased by fifteen percent. We can think of the full length as 100% of the length. If we take away fifteen percent, we are left with 100 – 15, or 85% of the length. In other words, the final length is 85% of the original length. We can represent 85% as a decimal by moving the decimal two places to the left.

l_{2} = 85% of l_{1} = 0.85l_{1}

Similarly, if we decrease the width by 20%, we are only left with 80% of the width.

w_{2} = 80% of w_{1} = 0.80w_{1}

We can now express the final area in terms of l_{1} and w_{1} by substituting the expressions we just found for the final length and width.

A_{2}=l_{2}\cdot w_{2} = (0.85l_{1})(0.80w_{1}) = 0.68l_{1}w_{1}

Lastly, let's apply the formula for percent of change, which will equal the change in the area divided by the original area. The change in the area is equal to the final area minus the original area.

percent change = \frac{(A_{2}-A_{1})}{A_{1}}(100%)

=\frac{(0.68l_{1}w_{1}-l_{1}w_{1})}{l_{1}w_{1}}(100%)

=\frac{-0.32l_{1}w_{1}}{l_{1}w_{1}}(100%) = –0.32(100%) = –32%

The negative sign indicates that the rectangle's area decreased. The change in the area was a decrease of 32%.

The answer is 32.

Example Question #1998 : Psat Mathematics

The cost of a load of laundry is reduced by 35\%. The cost is then reduced 2 weeks later by another 25\%. What is the overall reduction?

Possible Answers:

55\%

60\%

44\%

65\%

51\%

Correct answer:

51\%

Explanation:

The original reduction brings the total to 65\% of the original value. Taking a 25\% discount off that price gives 48.75\% of the original value. This means the reduction had been 51\%.

Example Question #3 : Percent Of Change

The cost of a shirt in January is  dollars. In February, the cost is decreased by 10%. In March, the cost is decreased by another 10%. By what percentage did the shirt decrease in total between January and March?

Possible Answers:

We must know the original cost to find the answer

Correct answer:

Explanation:

The best way to answer this question is to plug in a number for n. Since you are working with percentages, it may be easiest to use 100 for n.

We know that in the month of February, the cost of this shirt was decreased by 10%. Because 10% of 100 is $10, the new cost of the shirt is $90.

In March, the cost of the shirt decreased another 10%. 10% of 90 is 9, so the cost of the shirt is now $81.

To find the total percentage decrease, you must divide 81 by 100 and subtract it from 1.

1 – (81/100) = 1 – 0.81 = 0.19

The total decrease was 19%.

Example Question #1 : How To Find The Percent Of Decrease

The cost of a hat increases by 15% and then decreases by 35%. After the two price changes, the new price of the hat is what percent of the original?

Possible Answers:

80%

74.75%

88.91%

75%

85.3%

Correct answer:

74.75%

Explanation:

The easiest way to do percentage changes is to keep them all in one equation. Therefore, we would say that an increase of 15% is the same as multiplying the original value by 1.15. Likewise, we would say that a discount by 35% is the same as multiplying the original by .65.

For our problem, let the hat cost X dollars originally. Therefore, after its increase, it costs 1.15X dollars. Now, we can consider this new price as the whole to which the discount will be applied. Therefore, a 35% reduction is (1.15X) * 0.65.

Simplifying, we get 0.7475, or 74.75%.

Example Question #92 : Percentage

A dress is reduced in price by 35\%, but it still doesn't sell, so the manager discounts it by another 10\%. What is the total percentage discount?

Possible Answers:

None of the other answer choices are correct.

38.5\%

45\%

41.5\%

31.5\%

Correct answer:

41.5\%

Explanation:

For these type of questions, it is always best to pretend that we are beginning with a $100 item and to calculate from there.

100\times(1-0.35)=100\times0.65=65

65\times(1-0.1)=65\times 0.9=58.5

If an item that is $100 is discounted by 35%, and then another 10%, the new price is 58.5%.

100-58.5=41.5

The price difference (discount) is $41.5 for every $100, or:

\frac{41.5}{100}=0.415=41.5\%

The total discount is 41.5%.

 

Example Question #1 : How To Find The Percent Of Increase

The population of Town A is 12,979 people in 1995.  The population, when measured again in 2005, is 22,752.  What was the change in population to the nearest whole percentage point?

Possible Answers:

68%

43%

175%

57%

75%

Correct answer:

75%

Explanation:

Since we are looking for the change, we must take the

(Ending Point – Starting Point)/Starting Point * 100%

(22752 – 12979)/12979 * 100%

9773/12979 * 100%

0.753 * 100%

75%

Example Question #1 : Percent Of Change

A factory produced 2500 units during the month of September.  In order to increase production by 12% in the month of October, the factory hired more workers.  How many units were produced in October?

Possible Answers:

4000

2800

3000

3600

3200

Correct answer:

2800

Explanation:

This is a percentage increase problem. 

Easiest approach :  2500 x 1.12 = 2800

In this way you are adding 12% to the original.

Using the formula, find 12% of 2500

12/100 = x/2500, 

30000 = 100x

300 = x

Now add that to the original to find the new production:

2500 + 300 = 2800

Example Question #51 : Percentage

The radius of a given circle is increased by 20%.  What is the percent increase of the area of the circle.

Possible Answers:

44%

40%

20%

100%

144%

Correct answer:

44%

Explanation:

If we plug-in a radius of 5, then a 20% increase would give us a new radius of 6 (which is 1.2 x 5).  The area of the new circle is π(6)2 = 36π, and the area of the original circle was π(5)2 = 25π .  The numerical increase (or difference) is 36π - 25π = 11π.  Next we have to divide this difference by the original area: 11π/25π = .44, which multiplied by 100 gives us a percent increase of 44%.  The percent increase = (the numerical increase between the new and original values)/(original value) x 100. The algebraic solution gives us the same answer.  If radius r of a certain circle is increased by 20%, then the new radius would be (1.2)r.  The area of the new circle would be 1.44 π r2 and the area of the original circle πr2.  The difference between the areas is .44 π r2, which divided by the original area, π r2, would give us a percent increase of .44 x 100 = 44%.

Example Question #241 : Arithmetic

Phoenicia is a grocery store that is expanding quickly. 

In 2011 Phoenicia's total sales were $1,800,800.

In 2012 their sales rose to $2,130,346. 

By what percentage did the store increase its income from 2011 to 2012. 

(Round answer to the nearest tenth.)

Possible Answers:

18.3%

19.2%

21.0%

16.4%

10.5%

Correct answer:

18.3%

Explanation:

$1,800,800 divided by 100 equals 18,008 and $2,130,346 divided by 18,008 is 118.3

So we know that $2,130,346 is 118.3% of the sales in the previous year. Hence sales increased by 18.3%.

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