Consider the following case of completing first order reactions.
After the start of the reaction at t = 0 with only A, the [B] is equal to the [C] at all times. The time in which all three concentrations will be equal is given by
t=12K2ln3
K1=K2=23rd of A has reacted for [A] = [B] = [C]
∴K1+K2=1tln[A]013[A]0
t=1K1+K2ln3=12K1ln3=12K2ln3