Question 481420
Add their rates of working.
Warren's rate is: ( 1 tennis court ) / ( 6 hrs)
Dindo's rate is: ( 1 tennis court ) / ( 10 hrs )
Ermer's rate is: ( 1 tennis court ) / ( 15 hrs ) 
Let {{{ x }}} = the time in hours for all 3 painting together
to finish 1 tennis court.
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{{{ 1/6 + 1/10 + 1/15 = 1/x }}}
Multiply both sides by {{{ 30x }}}
{{{ 5x + 3x + 2x = 30 }}}
{{{ 10x = 30 }}}
{{{ x = 3 }}}
It will take all 3 working together 3 hrs to paint 1 tennis court