Question 1205672
Basic equation to relate the parts is as {{{n*r*x=j}}};
n, how many builders (or workers)
r, the work rate of any 1 worker
x, the time (in days, for this example)
j, how much work, or how many houses (being 1 for this example)


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8 builders can build a house in 40 days, ...
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{{{8r*40=1}}}


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how long will it take for 2 builders to build a house?
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you want to solve for value of x in
{{{2r*x=1}}}.


System is  {{{system(8*40*r=1,2rx=1)}}}
but you essentially will no longer need variable, r.


Divide equation 1 by equation 2.

{{{(8*40)/(2x)=1}}}

{{{2x(1/(2*4*40))=1}}}

{{{x/160=1}}}

{{{highlight(x=160)}}}{{{days}}}