<PRE><B>Question 1199624</B>
{{{(NUMBERofWORKERS)(WORKRATE)(TIME)=AMOUNTofWORK}}}


4 painters that have completed 12 rooms in 8 hours. 
{{{4*r*8=12}}}


how many hours does it take 7 painters to paint 21 rooms?
x, number of asked hours of time

{{{7rx=21}}}



Divide entire second equation by entire first equation, and solve for x.
The number r will simply not be needed (cancels in the operation).


{{{(7rx)/(4*8*r)=21/12}}}

{{{(7/(4*8))x=(7*3)/(4*3)}}}

{{{x=(4*8*7*3)/(4*3*7)}}}

{{{highlight(x=8)}}}</PRE>