Question 1045208
Alternative method is assign variables for each type of person's rate in units of JOB/DAYS, and this gives linear equations.


x for one-boy rate
y for one-man rate


{{{system((2y+5x)4=1,(4y+4x)3=1)}}}


{{{system(20x+8y=1,12x+12y=1)}}}



Elimination Method should be more comfortable to use for solving the system.
Multiply first equation by 3, and multiply second equation by 2.
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{{{system(60x+24y=3,24x+24y=2)}}}
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Subtract second equation from first equation, member by member.
{{{system((60x+24y)-(24x+24y)=3-2)}}}
{{{36x=1}}}
{{{highlight(x=1/36)}}}
Remember, here the unit of work rate is {{{JOBS/DAYS}}}, so this rate for ONE BOY means  "ONE JOB done in 36 DAYS".


You can use the same method for eliminating the x; but here, the best choice is return to {{{system(20x+8y=1,12x+12y=1)}}},  but multiply first equation by 3; and multiply second equation by 5.

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You do it......