设甲队需要用x天,乙队需要用y天,可列方程
x-y=30
20/x + 20/y = 1
解得x=60,y=30,舍去负根
即甲队需要60天,乙队需要30天.
(2)
(1 - a/60) / (1/60 + 1/30) = (1 - a/60) / (1/20) = 20 - (1/3)*a
设甲队需要用x天,乙队需要用y天,可列方程
x-y=30
20/x + 20/y = 1
解得x=60,y=30,舍去负根
即甲队需要60天,乙队需要30天.
(2)
(1 - a/60) / (1/60 + 1/30) = (1 - a/60) / (1/20) = 20 - (1/3)*a