Question 1030071
Crew A's rate of painting:
[ 1 room painted ] / [ 4 hrs ]
Crew B's rate of painting:
[ 1 room painted ] / [ 5 hrs ]
Let {{{ t }}} = time in hrs for A and B
to paint the room working together
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Add their rates of painting to get 
their rate of painting together
{{{ 1/4 + 1/5 = 1/t }}}
Multiply both sides by {{{ 20t }}}
{{{  5t + 4t = 20 }}}
{{{ 9t = 20 }}}
{{{ t = 2 + 2/9 }}} hrs
Convert hrs to minutes
{{{ (2/9)*60 = 13.333 }}}
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Working together they take 
2 hrs 13.333 min to paint the room
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check:
{{{ 1/4 + 1/5 = 1/t }}}
{{{ 1/4 + 1/5 = 1/(20/9) }}}
{{{ .25 + .2 = 9/20 }}}
{{{ .45 = .45 }}}
OK