SOLUTION: Working together, it takes two computers 4 minutes to send out a company's email. If it takes the slower computer 9 minutes to do the job on its own, how long will it take the fast

The rate of work of two computers working together is    of the job per minute.


The rate of work of the slower computer is    of the job per minute.


Hence, the rate of work of the faster computer is the difference   -  =  =   of the job per minute.


It means that the faster computer can complete the job in  minutes = 7 minutes and 12 seconds working alone.     ANSWER 

 
     When solving problems like this one, you should know and should remember two basic facts:

         - the rate of work of two workers/machines/tubes  is the sum of the rates of individuals;

         - If you are given the combined rate of work of two workers/machines/tubes and the rate of work of one individual, 
           then the rate of work of the other individual is the difference of rates.