Question 1193694
If you are really stuck, try setting up a data table based on RT=D.


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<pre>
                    SPEEDS           TIMES           DISTANCE

HOW HE DID           r                               40

WHAT IF              r+8                             40

DIFFERENCE                             16
</pre>
This and what to do with it should be clear enough.  Fill in what is missing and form the equation needed, and then solve.