1. After t years, $50e^{-0.015t}$ pounds of a deposit of a radioactive substance remain. The average amount per year not lost by radioactive decay during the second hundred years is
(A) 2.9 lb
(B) 5.8 lb
(C) 7.4 lb
(D) 11.1 lb
(E) none of these

if it helps the answer is B but i need help how to get that answer

2. Originally Posted by yoman360
After t years, $50e^{-0.015t}$ pounds of a deposit of a radioactive substance remain. The average amount per year not lost by radioactive decay during the second hundred years is
(A) 2.9 lb
(B) 5.8 lb
(C) 7.4 lb
(D) 11.1 lb
(E) none of these

if it helps the answer is B but i need help how to get that answer
The mass remaining at time $t$ is:

$m(t)=50e^{-0.015t}$

The amount lost between $t_1$ and $t_2$ is:

$M(t_1,t_2)=m(t_1)-m(t_2)$

So the average loss per year is:

$A(t_1,t_2)=M(t_1,t_2)/(t_2-t_1)$

CB