From B to C, the round-trip time ratio is 1/9: 1/8 = 8: 9.

The ratio of the total round-trip time is1112: 3/2 =11:18.

〔( 18- 1 1)-(9-8)〕/(3- 1)=3

Then the time ratio between AB and BC is (1 * 3): 8 = 3: 8.

It takes1112 * 3/(8+3) =1/4 (hours) to go to the AB section.

It takes1112-1/4 = 2/3 (hours) to go to BC.

Then the whole AC stroke is12 *1/4+9 * 2/3 = 9 (km).

2) The meeting time is 31.5 * 2/12 = 21/4 (hours).

Then the car travels 31.5/(21/4-4.5) = 42 (km) per hour.