Digital PSAT Math Practice Question 86: Answer and Explanation

Question: 86

Spread of a Computer Virus

The table above gives the number of computers infected with a virus. Which of the following functions models the number of computers infected, C(d), after d days?

• A. C(d) = 10^2d + 10(d - 1) + 1
• B. C(d) = 100 + 10^d
• C. C(d) = 100 + 10(d - 1) + 1
• D. C(d) = 100 + 10^{(d - 1)}

Correct Answer: D

Explanation:

(D) The easiest way to approach a problem like this is to test some points with each equation to see which equation works.

Choice (A):

So the equation works for day 1. Let's see if it works with day 2:

This doesn't match the number for day 2, so we can rule out choice (A).

Choice (B):

This isn't the right number for day 1, so we can rule out choice (B).

Choice (C):

This works, so let's try C(2):

This doesn't work, so we can rule out choice (C).

Choice (D):

Obviously, choice (D) is the correct answer.