math
posted by Mary .
Jack usually mows his lawn in 4 hours. Marilyn can mow the same in 5 hours. How much time would it take for them to mow the lawn together?

From the time it takes to do the complete joh, calculate the time it takes to do per hour. The resulting rate can then be added to find the time it takes for both of them.
1/(1/4+1/5)=1/(9/20)=20/9=2 2/9 hours. 
the time t that it takes for a salesman to drive a certain distance d varies inversely as the average speed r . it takes the salesman 4.75 h to travel between two cities at 60 mi/h . how long would the drive take ?