PSAT Math : Decimal Operations
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Example Questions
Example Question #1 : How To Multiply Decimals
45.728 x 3.2 = ?
14.63296
14632.96
146.3296
146329.6
1463.296
146.3296
Multiply the numbers out in long format, then move the decimal point over the total number of decimal points in the two numbers (3 in the first, 1 in the second, so 4 total):
45.728 * 3.2 = 91456
91456 + 1371840 = 1,463,296 → 146.3296
Or just count the decimal points in the answer and make sure it's 4.
Example Question #1 : How To Multiply Decimals
At the farmer's market, oranges are $0.30 each, apples are $0.25 each, bananas are $0.40 each, and tomatoes are $0.60 each. If Scott buys 3 oranges, 7 apples, 4 bananas, and 8 tomatoes, how much does he spend?
$7.75
$10.40
$6.80
$11.40
$9.05
$9.05
To find out how much Scott spends, we need to multiply the quantity of each fruit by its price and add them all up.
3 * $0.30 + 7 * $0.25 + 4 * $0.40 + 8 * $0.60 = $9.05
Example Question #2 : How To Multiply Decimals
If 
We can approximate the value of y pretty quickly:
y ≈ 1(0.50)(52) = 26
Since y = 0.25x = (1/4)x we know that x = 4y ≈ 4(26) = 104
Example Question #3 : How To Multiply Decimals
0.06
0.6
6
0.006
0.0006
0.006
It's much easier to work with whole numbers, so we'll ignore the decimals for the first step: 5 x 12 = 60
Now look at the decimals. To add them back in, think of 60 as 60.0. Now move the decimal point four places to the left: 0.5 x 0.012 = .0060 = .006
Example Question #3 : How To Multiply Decimals
At the store, green fabric costs $4.53 a yard. How much does 3.8 yards of green fabric cost? (Round to two decimal places.)
$17.21
$17.31
$17.12
$17.22
$17.13
$17.21
This is a simple multiplication problem involving decimals. Simply multiply 4.53 by 3.8:
Now, round to the nearest two decimal places, giving an answer of 17.21.
Example Question #4 : How To Multiply Decimals
Find the product. Round to the nearest hundredths place.
The hundredths place is the position two places to the right of the decimal. Our product is 
Example Question #1 : Decimal Operations
simplify: 0.2 ÷ 0.04
0.2 ÷ 0.04 <- easiest way to approach is to move the decimal to the right (the same amount of spaces on each side) so that the decimal disappears
since 0.04 has 2 spaces and 0.2 only has 1, move the decimal 2 spaces to the right on each side
0.2 ÷ 0.04 = 20 ÷ 4 = 5
Example Question #1 : Decimal Operations
If the cost of 20 equally priced pencils cost $5.80, then at the same rate what is the greatest number of pencils you can purchase for $50?
173
150
172
172.41
200
172
Remeber for this problem that you cannot spend more than $50. First find the price of each pencil by 5.80/20 = 29 cents. And then use this to do 50/0.29 = 172.41. So the greatest number of pencils you can purchase for $50 is 172 pencils.
Example Question #2 : Decimal Operations
Charlie has charges on his credit card of $63.70, $37.32, and $81.78. If he pays down half his bill at once, and then pays the rest in four equal monthly installments, how much is each monthly installment?
Adding all the charges together and dividing by 2 yields $91.40. Dividing this by 4 yields a monthly installment of $22.85.
Example Question #3 : Decimal Operations
Simplify:
Multiply both the numerator and denominator by some power of 10 to get rid of the decimals; in this case, multiply both by 10,000.
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