What will be the length of simple pendulum if it complete 1 vibration in l second?

What will be the length of simple pendulum if it complete 1 vibration in l second? 

Ans.Let the time taken by simple pendulum is 1 second

We know that, 

T=2π √l/g

T^2= 4 π^2 *l/g

l=T^2 g/4*(3.14) ^2

l= (1) ^2 *g/4*(3.14) 

l=9.8/(3.14) ^2 *4

l=0.25m