设甲X天,乙Y天,则
甲速度是1/X,乙速度是1/Y
10*(1/X+1/Y)=1 两边乘XY
10(X+Y)=XY
10X+10Y=XY
10X=Y(X-10)
Y=10X/(X-10)
则
1/X*12+1/Y*6=1
12/X+6(X-10)/10X=1 两边乘10X
120+6X-60=10X
10X-6X=120-60
4X=60
X=15天
Y=10X/(X-10)
=10*15/(15-10)
=30天