Question 885611
Let {{{ R[B] }}} = Baba's rate of stacking in cups/hr
Let {{{ R[L] }}} = Lala's rate of stacking in cups/hr
Let {{{ 200 }}} cups stacked = 1 job
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Add their rates of stacking to get rate stacking together
(1) {{{ R[B] + R[L] = 1 / ( 12/5 ) }}}
(1) {{{ R[B] + R[L] = 5/12 }}}
(1) {{{ 12R[B] + 12R[L] = 5 }}}
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In 1 hr, Baba stacked:
{{{ R[B] * 1 = R[B] }}} cups
The fraction of the job left to do is:
{{{ 1 - R[B] }}}
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(2) {{{ R[B] + R[L] = ( 1 - R[B] ) / ( 9/5 ) }}}
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Multiply both sides of (2) by {{{ 9/5 }}}
(2) {{{ ( 9/5 )*( R[B] + R[L] ) = 1 - R[B] }}}
(2) {{{ 9*( R[B] + R[L] ) = 5*( 1 - R[B] ) }}}
(2) {{{ 9R[B] + 9R[L] ) = 5 - 5R[B] ) }}}
(2) {{{ 14R[B] + 9R[L] = 5 }}}
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I can say:
(1) {{{ 12R[B] + 12R[L] = 14R[B] + 9R[L] }}}
(1) {{{ 3 R[L] = 2R[B] }}}
(1) {{{ R[L] = (2/3)*R[B] }}}
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Substitute (1) into (2)
(2) {{{ 14R[B] + 9R[L] = 5 }}}
(2) {{{ 14R[B] + (2/3)*9R[B] = 5 }}}
(2) {{{ 14R[B] + 6R[B] = 5 }}}
(2) {{{ 20R[B] = 5 }}}
(2) {{{ R[B] = 1/4 }}}
and
(2) {{{ R[L] = (2/3)*( 1/4 ) }}}
(2) {{{ R[L] = 1/6 }}}
Working alone, it takes Lala 6 hrs 
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check:
(1) {{{ R[B] + R[L] = 5/12 }}}
(1) {{{ 1/4 + 1/6 = 5/12 }}}
(1) {{{ 3/12 + 2/12  = 5/12 }}}
OK
also:
(2) {{{ R[B] + R[L] = ( 1 - R[B] ) / ( 9/5 ) }}}
(2) {{{ 1/4 + 1/6 = ( 1 - 1/4 ) / ( 9/5 ) }}}
(2) {{{ 5/12 = ( 3/4 )*( 5/9 ) }}}
(2) {{{ 5/12 = 5/12 }}}
OK