A man travelled a distance of 90Km in 9 hours partly on foot at 8 kmph and partly on bicycle at 17 ...........

Explanation:

Distance covered in 1 hour on foot = 8 km 
Distance covered in 1 hour on bicycle = 17 km
Average distance covered in 1 hour = 90/9 km   = 10km (mean distance)

Distance covered in        Distance covered in

1 hour on foot                  1 hour on bicycle

      (8 km)                                  (17 km)

                     \                       /
                     Mean Distance

                           (10 km)

                    /                        \

     (17 – 10)          :           (10 – 8)

            7                                   2 

Thus out of 9 hours, he took 7 hours on foot
Distance covered on foot  = (8×7)Km  =56 Km.