A farmer travelled a distance of 61km in 9hours. He travelled partly on foot @ 4km/hr and partly on bicycle @ 9km/hr.The distance travelled on foot is:

Answer is 

by bicycle9km/hr x5 =45 km

by foot      4km/hr x4 =16 km

so C.16 km is the answer

C.16 km.

If he walked for9 hours => distance =9 *4 =36 km, it is25 km less than61 km

if he rode bike => he will get additional (9-4) km distance for every hour. So he rode5 hours on bike, and walked (9-5) hours on foot. So he walked4 *4 =16 km.