Question 657668
<pre>
You might like this way better:
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Your rate is 1 window per 15 minutes, or {{{(1_window)/(15_minutes)}}}

Her rate is 1 window per 20 minutes, or {{{(1_window)/(20_minutes)}}}

Your and her combined rate is 12 windows per x minutes, or {{{12_windows/(x_minutes)}}}

    The equation comes from

             your rate + her rate = your combined rate

                          {{{1/15}}} + {{{1/20}}} = {{{12/x}}}
Multiply thru by LCD = 60x

                          4x + 3x = 720
                               7x = 720
                                x = 102 6/7 minutes or about 
1 hour 43 minutes.      

Edwin</pre>