B would have taken 10 hours more than what A would have taken to complete a task. Working together they can complete the task in 12 hours. How many hours would B take to do 50% of the task?

15

let A takes n hours, B will take (n+10) hours to do the same work.

together they can complete the same  work in 12 hours.

In 1 hour, A and B together do

\(\frac{1}{n}\)  +  \(\frac{1}{n + 10}\)  =  \(\frac{1}{12}\)

 12(n+10) + 12n = n(n+10)

on solving

n = 20. [Note: n = -6 is not possible ]

So A does the work in 20 hours and  B done in (20+10) = 30 days

if B does whole work in 30 days then 50% of work will be done in 15 days.