Two friends are walking by a cliff face when they notice their voices echo. One friend says to the other "It is 50 metres to that cliff. I can find out the speed of sound using this echo!" The first friend claps and listens for the echo. Then they keep clapping exactly in time with the echoes from the cliff. The second friend times how long it takes for the first friend to clap like this thirty times. It takes them nine seconds to clap thirty times in time with the echoes. Using their experiment and their data, calculate the speed of sound in air.