Question 272215
Distance(d) equals Rate(r) times Time(t) or d=r/t; r=d/t and t=d/r

Average Speed=(Total Distance)/(Total Time)
Total Distance = 100km
Total Time=Time for the 1st half + Time for the 2nd half
Time for the 1st half=50/60=5/6 hr
Time for the 2nd half=50/90=5/9 hr
So,
Average Speed=100/(5/6+5/9)
Average Speed=100/(15/18 +10/18)
Average Speed=100/(25/18) multiply numerator and denominator by 18/25
Average Speed=100(18/25)/1=4*18=72 km/hr


Hope this helps---ptaylor