Answer :
Man takes (13/15)*60=52 minutes to cover distance of 13 km.
Then in the remaining 48 minutes he has to cover 8 km i.e. (8/48)=1/6 km/min i.e 10 km/hr.
Given:
(i) The man cycles from A to B.
(ii) The total distance is 21 km and time taken is 1 hour 40 minutes
(iii) The road from A is level for 13km and then uphill to B
(iv) His average speed on level is 15 km/hr.
To find:
(i) Average uphill speed
Solution:
Total distance is 21 km.
Total time taken = 1 hour 40 mins
= 5/3 hours [Representing 40 mins in hours as 40/60 = 2/3]
= 100 minutes
Distance travelled on level road = 13km
Speed = 15km/hr.
So, time taken = Distance/Speed
= (13/15) hours
= 0.867 hours
= 52 minutes
So, distance of the uphill road = Total distance - Level road distance
= (21 - 13)km
= 8 km
Total time taken on uphill road = Total time - Time on level road
= (100 - 52)mins
= 48 mins
= 0.8 hours
So, average speed = Distance/Time
= (8/0.8) km/hr
= 10 km/hr
His average uphill speed is 10 km/hr.