Question 368453
Rate*Time=Output
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Let the output required to dig the garden equal Z.
{{{R*t=Z}}}
Calculate the father's rate,
{{{R[f]*3=Z}}}
{{{R[f]=Z/3}}}
Calculate the son's rate,
{{{R[s]*4=Z}}}
{{{R[s]=Z/4}}}
Putting their rates together, calculate the time, 
{{{(R[f]+R[s])*t=Z}}}
{{{(Z/3+Z/4)*t=Z}}}
{{{(4/12+3/12)*t=1}}}
{{{highlight(t=12/7)}}} hours