SOLUTION: Jonathan can type a 20 page document in 40 minutes, Susan can type it in 30 minutes, and Jack can type it in 24 minutes. Working together, how much time will it take them to type t

Let the answer be x minutes for all three working together
to type 1 document.

Make this chart:


                   number         time            rate 
                 of documents      in              in
                    typed        minutes     documents/minute
Jonathan alone
Susan alone
Jack alone
All working together

Fill in x for the time for all three working together.  Put 1 for 
the number of documents typed in all four cases.  Fill in the times 
that are given for each person working alone:

                   number         time            rate 
                 of documents      in              in
                    typed        minutes     documents/minute
Jonathan alone        1            40 
Susan alone           1            30
Jack alone            1            24 
All working together  1             x

Next fill in the rates in documents/minute by dividing the
number of documents typed by the time:

                   number         time            rate 
                 of documents      in              in
                    typed        minutes     documents/minute
Jonathan alone        1            40              1/40 
Susan alone           1            30              1/30  
Jack alone            1            24              1/24
All working together  1             x              1/x 


Use the equation:

 




Solve that and get x=10 minutes

Edwin