Nuclide A has a decay constant that is half that of nuclide B. Samples of both types

Question:

Nuclide A has a decay constant that is half that of nuclide B. Samples of both types start with the same number of undecayed nuclei, N. In terms of N, after two of A’s half-lives have elapsed,
(a) How many of type A have decayed?
(b) How many of type B have decayed?