Pendulum with adjustable lengths

[dipset24]

Homework Statement

Most grandfather clocks have pendulums with adjustable lengths. One such clock loses 10 min per day when the length of its pendulum is 30in. With what length pendulum will this clock keep perfect time.

Homework Equations

none

The Attempt at a Solution

I don't know where to begin.

 

[marcusl]

What is the equation for the oscillation frequency of a pendulum?