rate problems in maths

Question

If a takes 3 hours to lay 100 brick and B takes 4 hours to lay 150 bricks how long does it take them together to lay 850 bricks

Answer 1

A lays 33 1/3 per hour

B lays 37 1/2 per hour

together they lay 70 5/6 per hour

together they lay 850 in 850/(70 5/6) hours

= 850(6/425) = 12 hours