Question 1006498
A cistern can be filled by two pipes. The small pipe alone will take 24 minutes longer than the larger pipe to fill the cistern alone. The small pipe alone will take 32 minutes longer to fill the cistern alone when the two pipes are operating together. How long will it take for the larger pipe to fill the cistern alone?
<pre>Let time larger pipe takes be L
Then larger pipe can fill {{{1/L}}} of cistern in 1 minute
Smaller pipe can fill cistern in (L + 24) mins, or {{{1/(L + 24)}}} of cistern in 1 minute
Since smaller pipe takes 32 minutes longer to fill cistern than when both pipes are on, then the time it
takes to fill cistern when both are on = L + 24 – 32, or L – 8 minutes. Both can fill {{{1/(L - 8)}}} of cistern in 1 min
The following 1-minute RATES equation is thus formed: {{{1/L + 1/(L + 24) = 1/(L - 8)}}}
(L + 24)(L – 8) + L(L – 8) = L(L + 24) -------- Multiplying by LCD, L(L + 24)(L – 8)
{{{L^2 + 16L - 192 + L^2 - 8L = L^2 + 24L}}}
{{{L^2 + L^2 + 16L - 8L - 192 = L^2 + 24L}}}
{{{2L^2 + 8L - 192 = L^2 + 24L}}}
{{{2L^2 - L^2 + 8L - 24L - 192 = 0}}}
{{{L^2 - 16L - 192 = 0}}}
(L - 24)(L + 8) = 0
L, or time larger pipe takes = {{{highlight_green(24)}}} minutes	  OR       L = - 8 (ignore)