Question 850604
Add their rates of mowing to get
their rate mowing together
Jack's rate:
( 1 lawn mowed ) / ( 7 hrs )
Marilyn's rate:
( 1 lawn mowed ) / ( 5 hrs )
------------------------
Let {{{ t }}} = time in hours for them
to mow the lawn working together
{{{ 1/7 + 1/5  = 1/t }}}
Multiply both sides by {{{ 35t }}}
{{{ 5t + 7t = 35 }}}
{{{ 12t = 35 }}}
{{{ t = 2. 9167 }}}
{{{ .9167*60 = 55 }}}
It will take them 2 hrs and 55 min mowing together
check:
{{{ 1/7 + 1/5  = 1/t }}}
{{{ 1/7 + 1/5  = 1/2.9167 }}}
{{{ .14286 + .2 = .34285 }}}
{{{ .34286 = .34285 }}}
close enough