Question 1010035
Let {{{ t }}} = Dale's time in hrs
{{{ 72/60 = 1.2 }}}
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rate working together:
[ 1 stadium ] / [ 1.2 hrs ]
Chip's rate working alone:
[ 1 stadium ] / [ 3 hrs ]
Dale's rate working alone:
[ 1 stadium ] / [ t hrs ]
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Add their rates of working to get their rate 
working together
{{{ 1/3 + 1/t = 1/1.2 }}}
Multiply both sides by {{{ 1.2*3*t }}}
{{{ 1.2t + 1.2*3 = 3t }}}
{{{ 1.8t = 3.6 }}}
{{{ t = 2 }}}
It would take Dale 2 hrs working alone
check:
{{{ 1/3 + 1/2 = 1/1.2 }}}
{{{ .333 + .5 = .833 }}}
{{{ .833 = .833 }}}
OK