Question 1018421
You can add their rates of working to
get their rate working together.
Mary's rate can be expressed as:
[ 1 job ] / [ 7 hrs ]
Matt's rate is:
[ 3 jobs ] / [ 7 hrs ] 
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Note that this is exactly the same as:
[ 1 job ] / [ 7/3 hrs ]
it's just 2 ways to look at the same thing
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Let {{{ t }}} = time in hrs for them to complete
the leaf-raking job working together
Their rate working together is:
[ 1 job ] / [ t hrs ]
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I can say:
{{{ 1/7 + 3/7 = 1/t }}}
{{{ 4/7 = 1/t }}}
Multiply both sides by {{{ 7t }}}
{{{ 4t = 7 }}}
{{{ t = 7/4 }}} hrs
{{{ 7/4 = 1 + 3/4 }}}
1 hr 45 min
You did the same thing a little differently