Question 968661
Add their rates of filling to get rate
of filling together
Let {{{ t }}} = time in minutes for 
the hot water faucet to fill sink
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Rate for hot water faucet:
[ 1 sink filled ] / [ t min ]
Rate for cold water faucet:
[ 1 sink filled ] / [ 3.9 min ]
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{{{ 1/t + 1/3.9 = 1/3 }}}
Multiply both sides by {{{ 3*3.9*t }}}
{{{ 3*3.9 + 3t = 3.9t }}}
{{{ 11.7  = .9t }}}
{{{ t = 13 }}}
It takes 13 minutes for 
the hot water faucet to fill sink