Two simple pendulums of length 5m and 20 m respectively are given small linear displacement in one direction at the same time. They will again be in the phase when the pendulum of shorter length has completed.... oscillations

If t is the time taken by pendulums to come in same phase again first time after $t=0$      and 

$N_S$ = Number of oscillations made by shorter length pendulum with time period $T_S$

$N_L=$ Number of oscillations made by longer length pendulum with time period $T_L$

Then  $t=N_S{T}_S=N_L{T}_L$