I was considering the following question; 'A fire engine travelling at 70m/s drives past a van travelling at 30m/s. The speed of sound in air is 330m/s and the fire engine has a frequency of 200Hz when stationary. At what frequency does the van driver hear the siren?' I understand you have to use f(prime) = f((v+/-vd)/(v+/-vs)), however I find it quite difficult to decide whether to use the + or - in the calculation, when things are moving relative to one another. An explanation of when to use a + and when to use a - in each of the numerator/denominator would be most helpful. The answer is 180Hz. Thanks.