19. A painting job can be completed by Team $\mathrm{A}$ alone in 2.5 hours or by Team B alone in 75 minutes. On one occasion, after Team A had completed a fraction $\frac{m}{n}$ of the job, Team B took over immediately. The whole painting job was completed in 1.5 hours. If $m$ and $n$ are positive integers with no common factors, find the value of $m+n$.
19. Answer: 6
Suppose Team B spent $t$ minutes on the job. Then
$$
\frac{t}{75}+\frac{90-t}{150}=1 .
$$

Thus $t=60$ minutes and so Team A completed $\frac{30}{150}=\frac{1}{5}$ of the job. So $m+n=6$.