Question 879180
Rusty's rate:
( 1 horse washed ) / ( 1 hr )
Gary's rate:
( 1 horse washed ) / ( 2 hrs )
Working together:
( 1 horse washed ) / ( t hrs )
--------------------------
{{{ 1/1 + 1/2 = 1/t }}}
Multiply both sides by {{{ 2t }}}
{{{ 2t + t = 2 }}}
{{{ 3t = 2 }}}
{{{ t = 2/3 }}}
It would take them 2/3 hr working together
---------------------------------------
Jessica's rate:
( 1 horse washed ) / ( 1 hr )
The rate for Gary and Rusty was:
( 1 horse washed ) / ( 2/3 hr )
--------------------------
I can find a new time {{{ t }}} for 
all three working together:
{{{ 1/(2/3) + 1/1 = 1/t }}}
{{{ 3/2  + 1 = 1/t }}}
Multiply both sides by {{{ 2t }}}
{{{ 3t + 2t = 2 }}}
{{{ 5t = 2 }}}
{{{ t = 2/5 }}}
All 3 working together would take
2/5 hr