Line 14 has an inconsistency with the initial data. The equation on 13 gives A as a function of time, so A(0) = 40. But when we set t = 0 in that equation and solve for A(0) we get A(0) = 0.0002, not 40. Otherwise your derivation and work look correct, so the only interpretation I can come up with is that your initial data is flawed.

-Dan