PSAT Math : PSAT Mathematics

Study concepts, example questions & explanations for PSAT Math

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Example Questions

Example Question #1 : How To Find Compound Interest

Donna wants to deposit money into a certificate of deposit so that in exactly ten years, her investment will be worth $100,000. The interest rate of the CD is 7.885%, compounded monthly.

What should Donna's initial investment be, at minimum?

Possible Answers:

More information is needed to answer the question.

Correct answer:

Explanation:

The formula for compound interest is

where  is the initial investment,  is the interest rate expressed as the decimal equivalent,  is the number of periods per year the interest is compounded,  is the number of years, and  is the final value of the investment.

Set  (monthly = 12 periods), and , and evaluate :

The correct response is $45,569.99.

Example Question #1 : How To Find Compound Interest

Tom invests , in a savings account with an annual interest rate of . If his investment is compounded semiannually, how much interest does he earn after  years?

Possible Answers:

Correct answer:

Explanation:

In order to find the interest earned, used the compound interest formula

where  represents the number of times the account is compounded each year, and  represents the interest rate expressed as a decimal.

The account is worth $16882.63 after two years. Therefore Tom earns $1882.63 in interest.

Example Question #2 : Pattern Behaviors In Exponents

If ax·a4 = a12 and (by)3 = b15, what is the value of x - y?

Possible Answers:

-4

3

-9

-2

6

Correct answer:

3

Explanation:

Multiplying like bases means add the exponents, so x+4 = 12, or x = 8.

Raising a power to a power means multiply the exponents, so 3y = 15, or y = 5.

x - y = 8 - 5 = 3.

Example Question #1 : How To Find Patterns In Exponents

If p and q are positive integrers and 27= 9q, then what is the value of q in terms of p?

Possible Answers:

(3/2)p

p

3p

(2/3)p

2p

Correct answer:

(3/2)p

Explanation:

The first step is to express both sides of the equation with equal bases, in this case 3. The equation becomes 33p = 32q. So then 3p = 2q, and q = (3/2)p is our answer. 

Example Question #2 : How To Find Patterns In Exponents

Simplify 272/3.

Possible Answers:

9

125

3

27

729

Correct answer:

9

Explanation:

272/3 is 27 squared and cube-rooted. We want to pick the easier operation first. Here that is the cube root. To see that, try both operations. 

272/3 = (272)1/3 = 7291/3 OR

272/3 = (271/3)2 = 32

Obviously 32 is much easier. Either 32 or 7291/3 will give us the correct answer of 9, but with 32 it is readily apparent. 

Example Question #1 : How To Find Patterns In Exponents

If  and  are integers and 

 

what is the value of ? 

Possible Answers:

Correct answer:

Explanation:

To solve this problem, we will have to take the log of both sides to bring down our exponents. By doing this, we will get \dpi{100} \small a\ast log\left (\frac{1}{3} \right )= b\ast log\left ( 27 \right ).

To solve for \dpi{100} \small \frac{a}{b} we will have to divide both sides of our equation by \dpi{100} \small log\frac{1}{3} to get \dpi{100} \small \frac{a}{b}=\frac{log\left ( 27 \right )}{log\left ( \frac{1}{3} \right )}.

\dpi{100} \small \frac{log\left ( 27 \right )}{log\left ( \frac{1}{3} \right )} will give you the answer of –3.

Example Question #4 : How To Find Patterns In Exponents

If and , then what is ?

Possible Answers:

Correct answer:

Explanation:

We use two properties of logarithms: 

log(xy) = log (x) + log (y)

log(x^{n}) = nlog (x)

So

Example Question #571 : Algebra

Evaluate:

x^{-3}x^{6}

Possible Answers:

x^{6}

x^{9}

x^{-3}

x^{3}

x^{-18}

Correct answer:

x^{3}

Explanation:

x^{m}\ast x^{n} = x^{m + n}, here  and , hence .

Example Question #5 : How To Find Patterns In Exponents

Solve for

\left ( \frac{2}{3} \right )^{x+1} = \frac{27}{8}

Possible Answers:

None of the above

Correct answer:

Explanation:

\left ( \frac{2}{3} \right )^{x+1} = \frac{27}{8} = \left ( \frac{3}{2} \right )^{3} = \left ( \frac{2}{3} \right )^{-3}

  which means

Example Question #6 : How To Find Patterns In Exponents

Which of the following statements is the same as:

Possible Answers:

Correct answer:

Explanation:

Remember the laws of exponents. In particular, when the base is nonzero:

An effective way to compare these statements, is to convert them all into exponents with base 2. The original statement becomes:

This is identical to statement I. Now consider statement II:

Therefore, statement II is not identical to the original statement. Finally, consider statement III:

which is also identical to the original statement. As a result, only I and III are the same as the original statement. 

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