Question 845176
Add their rates of working to get
their rate working together
A's rate:
( 10 robots ) / ( 6 hrs )
B's rate:
( 10 robots ) / ( 4 hrs )
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Let {{{ t }}} = the time in hours for them
to build 10 robots working together
{{{ 10 / 6 + 10 / 4 = 10 / t }}}
Divide both sides by {{{ 10 }}}
{{{ 1/6 + 1/4 = 1/t }}}
Multiply both sides by {{{ 12t }}}
{{{ 2t + 3t = 12 }}}
{{{ 5t = 12 }}}
{{{ t = 2.4 }}}
Convert {{{ .4 }}} hrs to minutes 
{{{ .4*60 = 24 }}}
Working together, they take 2 hrs 24 minutes
to build 10 robots