Question 1030837
Rate of emptying for pipe A:
[  1 truck emptied ] / [ 3/4 hrs ]
Rate of emptying for pipe B:
[ 1 truck emptied ] / [ x hrs ]
Rate of emptying for both pipes together:
[ 1 truck emptied ] / [ (2/3)*x hrs ]
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Add rates for pipes A and B to get 
rate for both pipes together:
{{{ 1/((3/4)) + 1/x = 1/((2/3)*x) }}}
{{{ 4/3 + 1/x = 3/( 2x ) }}}
Multiply both sides by {{{ 6x }}}
{{{ 8x + 6 = 9 }}}
{{{ 8x = 3 }}}
{{{ x = 3/8 }}}
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check answer:
{{{ 1/(3/4) + 1/(3/8) = 1/(1/4) }}}
{{{ 4/3 + 8/3 = 4 }}}
{{{ 12/3 = 4 }}}
{{{ 4 = 4 }}}
OK