Calculus help please?

The fraction of Type A and Type B of hereditary genes in a population is A(t) and B(t), respectively, so that A(t) + B(t)=1. Initially, there were only A genes present. The A genes mutate into the B genes at a rate equal to 0.1% of the number of A genes present, and the B genes mutate into the A genes at a rate equal to 0.5% of the number of B genes present.