Question

A man drives from his home at 30 miles per hour to a shopping mall that is 20 miles from his home. On the return trip, he encounters heavy traffic and averages 12 miles per hour. To the nearest mile per hour, what is his average speed for the round-trip to and from the mall?

Answer 1

First half: 30 miles per hour for 20 miles. Time taken:

$d=st \iff t=\dfrac{d}{s}=\dfrac{30}{20}=1.5$

Second half: 12 miles per hour for 20 miles. Time taken:

$d=st \iff t=\dfrac{d}{s}=\dfrac{30}{12}=2.5$

In total, he traveled 60 (30+30) miles in 4 hours (1.5+2.5), for an average speed of

$s=\dfrac{d}{t}=\dfrac{60}{4}=15$