Question 840590
A car travels the first one-third of a certain distance with a speed of 10 km/hr,
 the next one-third distance with a speed of 20 km/hr
 and the last one-third distance with a speed of 60 km/ hr. 
The average speed of the car for the whole journey is
:
let a = the average speed for the trip
write a time equation; time = dist/speed; assume the total dist = 3
{{{1/10}}} + {{{1/20}}} + {{{1/60}}} = {{{3/a}}}
multiply eq by 60a, cancel the denominators and you have
6a + 3a + a = 60(3)
10a = 180
a = 180/10
a = 18 km/hr is the average for the trip