B) 45

C) 40

D) 30

A and B together can complete the work in 15 days, so (A+B)’s one day work = 1/15

If A alone can complete a work in 20 days then A’s one day work =1/20

Let B alone can complete the work in x days , so B’s one day work = 1/x

Now ,

(A+B)’s one day work = A’s one day work + B’s one day work

=> 1/15=1/20+1/x

=> 1/x= 1/15 – 1/20

=> 1/x = 1/60

=> x=60

Hence B alone can complete the work in 60 days

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