Time period of a pendulum,
T∝l
⇒T2T1=l2l1
If n is the number of vibrations after which the pendulums are again in phase, the number of vibration of the longer pendulum will be (n−1). Therefore,
T2×nT1×(n−1)=1 ⇒100(n)121(n−1)=1
⇒11n−11=10n ⇒n=11