1、
1÷(1/8+1/10)
=1÷9/40
=(4又4/9)小时
2、
[1-(1/12+1/15)×2]÷1/15
=[1-3/20×2]÷1/15
=[1-3/15]÷1/15
=4/5÷1/15
=12小时