There is a little mistake in the indices of the equation you derived, you brought back the minus sign after changing -ln(1/2) to ln(2) so the equation is

The 40% is related to number of atoms not time, if it decays to 40% of its original size after a time T then 0.4*N(0)= N(T)

Using the equation you derived,

The N(0) cancels on both sides.