How do I compute this physics problem about a bat and a moth?

Suppose a bat tries to locate a moth by emitting sound pulses and detecting the echo. Assume that the ultrasound frequency is 10 MHz. If the echo returns after 0.25 s, how far is the moth from the bat? I'm sure this is easy, but I have been racking my brain for a solution, we went over the doppler effect in class but this doesn't seem to fit the formula.