Question 864629
The student's rate is:
( 1 test finished ) / ( 40 min )
The teacher's rate is:
( 1 test finished ) / ( 40/3 min )
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Let {{{ t }}} = time in minutes to finish test
with both working on it together
Their rate working together:
( 1 test finished ) / ( t min )
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Add their rates to get rate working together
{{{ 1/40 + 3/40 = 1/t }}}
Then you would multiply both sides by {{{ 40t }}}
{{{ t + 3t = 40 }}}
{{{ 4t = 40 }}}
{{{ t = 10 }}} min