Question

A source of wave produces 40 crests and 40 troughs in 0.4 seconds. What is the frequency of the wave?

Answer 1

frequency is number of cycles per second.A cycle consist of a trough and crust.
Here 40 crust and trough occured in 0.4 second. ie 40 cycles in 0.4 second.

0.4 x 2.5=1 second
so cycles per second,frequency=40 x 2.5=100 Hz
(divide number cycle byn time, ie,40/0.4=100 Hz)