Question 1034413
Add their rates of working to get 
their rate working together
Mr. Jansen's rate:
[ 1 roof ] / [ 10 hrs ]
Mr. Doyle's rate:
[ 1 roof ] / [ 8 hrs ]
The 3 men working together:
[ 1 roof ] / [ 3.5 hrs ]
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Let {{{ t }}} time in hrs for Mr. Davies to 
shingle the roof working alone
{{{ 1/10 + 1/8 + 1/t = 1/3.5 }}}
{{{ 1/10 + 1/8 + 1/t = 2/7 }}}
Multiply both sides by {{{ 7*8*10t }}}
{{{ 56t + 70t + 560 = 2*80t }}}
{{{ 126t + 560 = 160t }}}
{{{ 34t = 560 }}}
{{{ t = 16.4706 }}} hrs
and
{{{ .4706*60 = 28.2353 }}}
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Working alone, Mr Davies takes 16 hrs 28 min
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check:
{{{ 1/10 + 1/8 + 1/16.4706 = 1/3.5 }}}
{{{ .1 + .125 +  .06071 = .28571 }}}
{{{ .225 + .06071 = .28571 }}}
{{{ .28571 = .28571 }}}
OK