Calculus word problem?

Question

This has been stumping me.

When a camera flash goes off, the batteries immediately begin to recharge the flash's capacitor, which stores electric charge given by the following. (The maximum charge capacity is M and t is measured in seconds.)
Q(t) = M(1 - e^(-t/a) )

a. find the inverse

b. How long does it take to recharge the capacitor to 74% of capacity if a = 9?

Answer 1

a)♣ thus Q/M = 1-exp(-t/a) → 1-Q/M =exp(-t/a) →
→ ln(1-Q/M) = -t/a → t =-a*ln(1-Q/M);
b)♦ t= -9*ln(1-0.74)= 9*ln(100/26) =12.12 s;

Answer 2

ok... and the question is?