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 co…

Let A would have taken x hours to complete the task,

⇒ According to given condition B would take (x + 10) hours to complete the task

⇒ Work done in one day by A and B will be 1/x and 1/(x + 10) units

Since working together they take 12 hours to complete the task,

⇒ 1/x + 1/(x + 10) = 1/12

⇒ (2x + 10)/(x2 + 10x) = 1/12

⇒ 24x + 120 = x2 + 10x

⇒ x2 - 14x - 120 = 0

⇒ (x - 20) (x + 6) = 0

⇒ x = 20, -6

∴ B will take 30 hours to complete the task and 15 hours to complete 50% task.