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

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