What is the length of a simple pendulum which ticks second?

Show that the motion of a simple pendulum is simple harmonic and hence derivean equation for its time period. Find the length of a simple pendulum which ticks seconds. (g=9.8ms^(-2))

What is the length of a seconds pendulum?

What is the length of second's pendulum ?

The length of a second pendulum is

The length of a second pendulum is

At a particular place, a simple pendulum of length 1m, makes 400 oscillations in 13 minute. What is the length of a second's pendulum at this place ?