You are riding your bicycle directly away from a stationary source of sound and hear a frequency that is 2.5% lower than the emitted frequency. The speed of sound at the temperature that day is 339 m/s. What is your speed?

f = f0(v + vo)/v.

Here, vo is the unknown. f = fo*.985
V is 339

1+vo/339 = .985
vo/330=-.025

solve for vo. the negative sign means moving away.