Question 666907
Add their rates of working to get their rate of working together
Let Mo's rate is ( 1 puzzle done ) / ( t hours )
Curly's rate is ( 2 puzzles done ) / (  t hours )
Larry's rate is ( 2 puzzles done ) / ( t hours }
---------------
{{{ 1/t + 2/t + 2/t = 1/40 }}}
Multiply both sides by {{{ 40t }}}
{{{ 40 + 80 + 80 = t }}}
{{{ t = 200 }}}
{{{ 2t = 400 }}}
Mo took 200 hrs to assemble the puzzle
check:
{{{ 1/200 + 2/200 + 2/200 = 1/40 }}}
{{{ 5/200 = 1/40 }}}
{{{ 1/40 = 1/40 }}}
OK