Or the solution of the equation: let the speed of a be x and the speed of b be y.

20x= 12*(x+y)

Y=2/3x。

The total number of items is 20x.

So the time required for B is 20x/y=20x/(2/3x)=30 hours.

Or: the cooperation efficiency of Party A and Party B =1÷12 =112; A efficiency =120 =1/20.

Efficiency =112-1/20 =1/30.

B Time required to complete it alone = 1÷ 1/30=30 hours.