<PRE><B>Question 625647</B>
Rewrite to show how I understand the problem
:
A does a work in 20 days. A worked for 28 days. 
Then B came. Working together, they take 76 days to complete 7 jobs.
 Find how many days it takes B to complete a work.
:
let b = time for B to do one job by himself
let one completed job = 1
:
{{{((28+76))/20}}} + {{{76/b}}} = 7
{{{104/20}}} + {{{76/b}}} = 7
multiply by 20b
104b + 20(76) = 20b(7)
104b + 1520 = 140b
1520 = 140b - 104b
1520 = 36b
b = {{{1520/36}}}
b = 42.22 days to do one job alone

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