Question 1087237
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...robert and william 12 hours to complete a history project. If Robert works alone on the same project for 16 hours and let william take over, william needs another 7 hours to complete it. 
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{{{system(1/r+1/w=1/12,(1/r)16+(1/w)7=1)}}}




BETTER APPROACH:
If use rates just as variables, not in a form of ratioinal numbers with times as denominators,...
{{{system(R+W=1/12,16R+7W=1)}}}
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{{{R=1/12-W}}}

{{{16(1/12-W)+7W=1}}}

{{{16/12-16W+7W=1}}}

{{{4/3-9W=1}}}

{{{4/3-3/3=9W}}}

{{{1/3=9W}}}

{{{highlight(W=1/27)}}}


William could do the whole job alone in 27 hours.