Calcuate the frequency of a wave when a stone is dropped into a tub of 15cm of water

you have to get a tub filled with 15cm of water. place a floating cork on the water. while holding a 30cm ruler upright drop a stone into water
from a hight of 15cm above water and see how many waves per minute and record the amount of waves and calcuate the frequency of wave and also record the hight of each wave