A couple more System of Equation problems that are stumping me!

This one I have tried 3 times, and still can not come up with a answer.

A manufacturing plant is going to use two different stamping machines to complete an order of 975 units. One produces 100 units per hour, while the other produces 75 units per hour. How long must each machine operate to complete the order if the faster machine needs to be shut down for two and one half hours for repairs?

Another one that is giving me fits is the following:

2 Cars are to start at the same place in Knoxville and travel in opposite directions. (Assume they drive in a straight line) The drivers know that one driver drives an average of 5mph faster than the other . They have agreed to stop and call each other after driving for 3 hours. In the telephone conversations they realize that they are 355 miles apart. What was the average speed of each driver?