An observer moves towards a stationary source of sound, with a velocity one-fifth one-fourth of  the velocity of sound. What is the percentage increase in the apparent frequency?

When observer moves towards stationary source then apparent frequency 
 $n^{\text{'}}=\left[\frac{v+v_{ }}{v}\right]n=\left[\frac{v+v/5}{v}\right]n=\frac{6}{5}n=1.2n$
   Increment in frequency = 0.2n so percentage change in frequency $=\frac{0.2n}{n}\times 100=20\%$.