Answer:
2 2/5 hours
Explanation:
In 6 hours Brian can lay 1 slab of concrete
dividing both side by 6
in 6/6 hours Brian can lay 1/6 slab of concrete
Thus, in 1 hour Brian can lay 1/6 slab of concrete
In 4 hours Greg can lay 1 slab of concrete
dividing both side by 4
in 4/4 hours Greg can lay 1/4 slab of concrete
Thus, in 1 hour Greg can lay 1/4 slab of concrete
Thus, total part of slab laid by both in 1 hour when they work together
1/6 + 1/4 = 4+6/(6*4) = 10/24 = 5/12
5/12 of slab of concrete is laid by both of them in 1 hour
time taken to lay 5/12 of slab = 1 hour
dividing both side by 5/12
time taken to 5/12/ 5/12 of slab = 1/5/12 hour = 12/5 hours
time taken to 1/1 of slab = 12/5 hours = 2 2/5 hours
Thus,
it takes 2 2/5 hours to lay the full slab of concrete when Brian and Greg work together,