Waves

A person driving car at a constant speed of \(15 \mathrm{~m} / \mathrm{s}\) is approaching a vertical wall. The person notices a change of \(40 \mathrm{~Hz}\) in the frequency of his car's horn upon reflection from the wall. The frequency of horn is _______________ \(\mathrm{Hz}\).

(Given: Speed of sound : \(330 \mathrm{~m} / \mathrm{s}\) )