Since N=32-32e-0.02t, you are asking when does N reach 32. Strictly speaking, it never does (if you put t=1000000 in there, you get a number very close to, but not quite, 32).
So the real problem is deciding when N is "close enough" to 32. That's a judgment call. Let's say that 31.5 is "close enough". Then the problem becomes this:
31.5 = 32-32e-0.02t
which you can re-arrange to
(1/64) = e-0.02t
Now to solve for t you need to get it out of the exponent. You can do that by taking the logarithm of both sides.
Hope this helps,
Stephen La Rocque.