Question 754882
Add the rate of filling and subtract the
rate of emptying
The rate of filling is:
( 1 tank filled ) / ( 60 min )
The rate of emptying is:
( 1 tank empty ) / ( 50 min )
Let {{{ t }}} = time to empty tank
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{{{ 1/60 - 1/50 = -1/t }}}
( note that the right side is negative because it
is an emptying rate )
Multiply both sides by {{{ 300t }}}
{{{ 5t - 6t = -300 }}}
{{{ -t = -300 }}}
{{{ t = 300 }}}
The tank is emptied in 300 min
{{{ 300/60 = 5 }}} 
or, 5 hrs
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check:
{{{ 1/60 - 1/50 = -1/300 }}}
{{{ .01667 - .02 = -.00333 }}}
{{{ -.00333 = -.00333 }}}
OK