Question 409722
Let {{{t}}} = time for both pipes to fill 3/4 of the pool
given:
{{{R[1] = 1/6}}} 1 pool/6 hours
{{{R[2] = 1/9}}} 1 pool/9 hours
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{{{R[1] + R[2] = (3/4)/t}}}
{{{1/6 + 1/9 = (3/4)/t}}}
Multiply both sides by {{{18t}}}
{{{3t + 2t = (3*18)/4}}}
{{{5t = 27/2}}}
{{{t = 2.7}}} hr
The 2 pipes together take 2.7 hr, or 2 hrs and 42 min
to fill 3/4 of the pool