Problem: a car is moving away from a stationary observer at 25ms^-1. It emits a frequency of 810Hz and the observer hears an apparent frequency of 762Hz. What is the speed of sound in air at that time?
The equation for a source moving away is
f' = f(v/(v+us)) where us is speed of source.
The Attempt at a Solution
There are 2 v's and I need just one, I have no idea where to start, please can you help?