+12 On his outward journey, Ali travelled at a speed of skm/h for 2 1/2 hours. On his return journey he increased his speed by 4 km/h and saved 15 minutes. Find Ali's average speed for the whole journey.
+129850 Rate * Time = Distance
Call the outward rate = S
Call the inward rate S+ 4
So
S( 2.5 hrs) = (S + 4) (2.25 hrs)
2.5 S = 2.25S + 9
2.5S - 2.25S = 9
.25S = 9
S = 9 / .25 = 36 km/h
S + 4 = 40km/h
The avg rate = 2 ( product of the rates) / ( sum of the rates) =
2 (36 * 40) / ( 36 + 40) =
2 (1440) / ( 76) ≈ 37.9 km/h
