Kindly tell with explaination.

Dear Student
Speed from A to B = 30 KM/h
Let the distance from A to B be D.

T i m e t a k e n t o t r a v e l f r o m A t o B T_{1} = \frac{D i s \tan c e t r a v e l l e d}{s p e e d} = \frac{D}{30} S p e e d f r o m B t o A i s 20 k m / h r T i m e t a k e n t o t r a v e l f r o m B t o A T_{2} = \frac{D i s \tan c e t r a v e l l e d}{s p e e d} = \frac{D}{20} T o t a l t i m e t a k e n T = T_{1} + T_{2} = \frac{D}{30} + \frac{D}{20} = \frac{D}{12} T o t a l d i s \tan c e = D + D = 2 D A v e r a g e s p e e d = \frac{T o t a l d i s \tan c e}{T o t a l t i m e} = \frac{2 D}{\frac{D}{12}} = 24 k m / h

Regards