Explanation:

let time taken by A to complete the work=x

let time taken by B to complete the work=x+20

total work=x(x+20)

A's efficienciency=x+20

B's efficiency=x

time taken by A and B together=x(x+20)/(2x+20)=24

x²-28x-480=0

x=40,-12

so time taken by A to complete the work=40