Question 657668
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If you can do one whole job in *[tex \LARGE x] time periods, you can do *[tex \LARGE \frac{1}{x}]th of the job in one time period.


So, specifically for your problem, you can do *[tex \LARGE \frac{1}{15}]th of a window in one minute, and sister can do *[tex \LARGE \frac{1}{20}]th of a window in one minute.  Working together you can do *[tex \LARGE \frac{1}{15}\ +\ \frac{1}{20}] of a window in one minute.


A little algebra music:


*[tex \LARGE \ \ \ \ \ \ \ \ \ \ \frac{1}{15}\ +\ \frac{1}{20}\ =\ \frac{4}{60}\ +\ \frac{3}{60}\ =\ \frac{7}{60}]ths


of a window in one minute, so the two of you working together can do one window in *[tex \LARGE \frac{60}{7}] or a little more than 8 and a half minutes.


Multiply by 12 to get your time to do all 12 windows.


John
*[tex \LARGE e^{i\pi}\ +\ 1\ =\ 0]
My calculator said it, I believe it, that settles it
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