Question 996069
 Jim and his dad are bricklayers. Jim can lay bricks for a fireplace and chimney in 5 days. 
With his father's help he can build it in 2 days.
 How long would it take his father to build it alone? 
:
let d = time required by Dad working alone
let the completed job = 1
:
A typical "shared work" equation
{{{2/5}}} + {{{2/d}}} = 1
multiply equation by 5d
5d*{{{2/5}}} + 5d*{{{2/d}}} = 5d*1
Cancel the denominators and we have
2d + 5(2) = 5d
10 = 5d - 2d
10 = 3d
d = 10/3
d = 3{{{1/3}}} days, Dad alone