The amount of a radioactive substance decreases by 10% every

12 hours. Currently the amount of substance is 100 grams. If n is

a whole number, which expression represents the number of

grams of substance left n days from now?

A. 100 (0.81) ^n

B. 100 (1.1) ^2n

C. 100 (0.81) ^n/2

D. 100 (0.8) ^n

Step-by-step explanation:

We would apply the formula,

y = ab^t

Where

a represents the initial amount of substance.

t represents the decay time.

From the information given

a = 100

t = 12 hours

Since after 12 hours, the amount of substance reduces by 0.1, then

y = 0.1 * 100 = 10

Therefore

10 = 100 * b^12

Dividing through by 100, it becomes

0.1 = b^12

Raising both sides of the equation by 1/12, it becomes

0.1^ (1/12) = b^12/12

b = 0.8

The equation becomes

y = 100 (0.8) ^t

Where t = n, the expression becomes

y = 100 (0.8) ^n