Question

By working together, A and B can finish a work in
15 days. If B alone can finish the work in 20 days,
in how many days can A alone finish the work?​

Answer 1

A and B → 15 days

A → 20days

So, let's take LCM of 15 and 20. That's 60.

Remember: This 60 unit is the total work to be done.

So, A and B do (60/15) = 4 unit work/day …(1)

And, B does (60/20) = 3 unit work/day …(2)

(1)-(2) gives

A does (4–3)= 1 unit/day

So, Total days req. = (Total work)/unit per day

i.e, A can do it in (60/1)=60 days