Two substances A(t_1/2 = 5min) and B(t_1/2 = 15min) are taken in such a way that initially [A] = 4[B — AITS & Test Series Chemistry Question

Question

Two substances A(t_1/2 = 5min) and B(t_1/2 = 15min) are taken in such a way that initially [A] = 4[B]. The time after which both the concentration will be equal is: (Assume that reaction is first order)