Question 882537
<pre>
                                        | percentage |     cups of
                                        | expressed  |  grapefruit juice
                       | cups of liquid | as decimal | contained in liquid
-----------------------|----------------|------------|--------------------
  5% grapefruit juice  |      40        |    .05     |    .05(40) = 2    
100% grapefruit juice  |       x        |   1.00     |      1.00x = x
-----------------------|----------------|------------|-------------------- 
     punch mixture     |    40+x        |    .10     |  .10(40+x) = 4+.10x

Equation comes from right-most column:

                                 2 + x = 4+.10x
                              20 + 10x = 40 + x
                                    9x = 20
                                     x = {{{20/9}}}
                                     x = {{{2&2/9}}} cups

Edwin</pre>