Let p be the speed of man in kmph
According to the given data in the question,
Distance travelled by bus in 10 min with 20 kmph == Distance travelled by man in 8 min with (20 + p) kmph in opposite direction
=> 20 x 10/60 = 8/60 (20 + p)
=> 200 = 160 + 8p
=> p = 40/8 = 5 kmph.
