Pipe A runs 30 liters of water per minute into a pool that

AAPL wrote:Economist GMAT

Pipe A runs 30 liters of water per minute into a pool that has a total volume of 3,600 liters. Pipe B fill a third of the pool in 6 hours. Working together how long will it take both pipes to fill the pool?

A. 2 hours and 36 minutes
B. 1.5 hours
C. 9 hours and 12 minutes
D. 15 hours
E. 1 hour and 48 minutes

$$?\,\,\,:\,\,\,{\rm{time}}\,\,{\rm{together}}\,\,{\rm{for}}\,\,3600\,\,{\rm{liters}}$$

Let´s use UNITS CONTROL, one of the most powerful tools of our method!

$$\left. \matrix{
A\,\,:\,\,\,{{30\,\,{\rm{liters}}} \over {\,1\,\,{\rm{minute}}\,}}\,\,\,\, \Rightarrow \,\,\,\,{{90\,\,{\rm{liters}}} \over {\,3\,\,{\rm{minutes}}\,}} \hfill \cr
B\,\,:\,\,\,\,{{1200\,\,{\rm{liters}}} \over {\,6\,\,{\rm{hours}}\,}}\,\,\,\left( {{{1\,\,{\rm{hour}}} \over {\,60\,\,{\rm{minutes}}\,}}} \right)\,\, = \,\,{{10\,\,{\rm{liters}}} \over {\,3\,\,{\rm{minutes}}\,}}\,\,\, \hfill \cr} \right\}\,\,\,\,\,\, \Rightarrow \,\,\,\,?\,\,\, = \,\,\,3600\,\,{\rm{liters}}\,\,\,\,\left( {{{3\,\,{\rm{minutes}}} \over {\,90 + 10\,\,{\rm{liters}}\,}}} \right)\,\,\, = \,\,\,108\min \,\,\, = \,\,\left( {60 + 48} \right)\,\,\min $$


This solution follows the notations and rationale taught in the GMATH method.

Regards,
Fabio.