Question 97176
Doy and his son can do a job in 4 1/2 days. After that they have worked for 3 days, Doy got sick and his son finished the work in 6 more days. How many days are required for each of them to do the work?

:
Let the completed job = 1
:
Let S = sons time alone
:
Let D = Doy's time alone
:
Working together for 3 days they would have completed {{{3/4.5}}} of the job
:
The boy worked alone for 6 days so he completed {{{6/S}}} of the job
:
Find S
{{{3/4.5}}} + {{{6/S}}} = 1
:
Multiply equation by 4.5S to get rid of the denominators, resulting in:
3s + 4.5(6) = 4.5s
3s + 27 = 4.5s
27 = 4.5s - 3s
1.5s = 27
s = 17/1.5
s = 18 days to complete the job, the son working alone
:
Find the time required by Doy working alone, using the "working together: equation:
{{{4.5/D}}} + {{{4.5/18}}} = 1
:
Multiply equation by 18D to get rid of the denominators, results:
4.5(18) + 4.5D = 18D
81 + 4.5D  = 18D
81 = 18D - 4.5D
13.5D = 81
D = 81/13.5
D = 6 days to complete the job, Doy working alone
:
:
Did this make sense to you?