## Physics Technology Update (4th Edition)

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