Question 356789
Rate*Time=Output
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Let {{{Y}}} be the output required to rake the entire yard. 
Find each person's rate.
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Luke:
{{{R[L]*3=Y}}}
{{{R[L]=Y/3}}}
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Don:
{{{R[D]*5=Y}}}
{{{R[D]=Y/5}}}
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Luke and Don together:
{{{(R[L]+R[D])*t=Y}}}
{{{(Y/3+Y/5)*t=Y}}}
{{{(5/15+3/15)*t=1}}}
{{{(8/15)*t=1}}}
{{{highlight(t=15/8)}}}hours