Answer: a. 85km/hr b.82.3km/hr
c. 84km/hr
Explanation: first let take the total time from San Antonio to Houston to be 2hr.
Half time 1hr was covered with speed of 72km/hr
Distance = speed*time=72km/hr *1hr
=72km
So too with the second half of 1hr covered with speed of 98km/hr
Distance = 98km
Total distance from Houston to San Antonio is 98+72 =170km
a. Average speed from San Antonio to Houston is
S1 =170/2
=85km/hr
b.half distance from Houston to San Antonio which is 170km/2
= 85km was covered with speed of 72km/hr first half, so time
t = dist/speed
t = 85/72 = 1hr 12 mins
Remaining 85 km covered with a speed of 98km/hr
Time = 85/98 = 0.88*60min
= 52 mins
Total time = 1hr +12mins +52mins
=2hr4mins= 124/60 hr
So average speed = distance/time
=170/124/60
Using reciprocal law
Average speed S2= 170*60/124
= 82.3km/hr
C. Average speed to and fro(entire tripe)
= (85+82.3)/2
=84km/hr
