B would have taken 10 hours more than what A would have taken to complete a task if each of them worked alone. Working together they can complete the task in 12 hours. How many hours would B take to do 50% of the task?
Show answer & explanation
Correct answer: B
Explanation
The correct answer is option 2. i.e. 15.
Time taken by A to complete a work = 't' hours
∴ B will take = (t + 10) hrs.
⇒ t2 – 14t – 120 = 0
⇒ t = 20
Time of A = 20 hrs., Time of B = 20 + 10 = 30 hrs.
To do 50% of task, B needs = 
= 15 hrs.