#### Answer

(a) The average speed is less than 20 m/s.
(b) More time is spent at 15 m/s than at 25 m/s.

#### Work Step by Step

$t_{1}$: time spent at 25 m/s.
$t_{2}$: time spent at 15 m/s.
$10~km=10,000~m$ and $20~km=20,000~m$
$t_{1}=\frac{10,000~m}{25~m/s}=400~s$
$t_{2}=\frac{10,000~m}{15~m/s}=670~s$
$t_{1}+t_{2}=1070~s$
$average~speed=\frac{20,000~m}{1070~s}=19~m/s\lt20~m/s$