Explanation:
we simply need to get the total distance and the total time and put them into relation, norming it to distance in 1 hour. after all, speed is always an average number over a period of time (even if it is a tiny period of time).
the total distance is
72 + 69 = 141 km
he drove 69 km going 60 km/h.
how much time did he need ?
we need to get the hours out of an expression of km and h.
so, we need to do
69 km × 1/60 km/h = 69 h / 60 = 69 minutes = 1.15 hours =
= 1 h 9 minutes
so, the total time was
48 + 69 = 117 minutes = 117/60 = 1.95 hours
the average speed for the whole trip was then
141 km / 1.95 h = 141/1.95 km / 1 h = 72.30769231... km/h