A can do a certain work in the same time in which B and C together can do it.If A and B together could do it in 20 days and C alone in 60 days ,then B alone could do it in:

(A+B)'s 1 day's work=1/20 

C's 1 day work=1/60 

(A+B+C)'s 1 day's work= 1/20 + 1/60 = 1/15

Also A's 1 day's work =(B+C)'s 1 day's work 

Therefore,  we get: 2 x (A's 1 day 's work)=1/15 

=>A's 1 day's work=1/30 

Therefore, B's 1 day's work= 1/20 - 1/30 = 1/60

So, B alone could do the work in 60 days.