What strategies can i use to add or subtract ?

Ineed to know the answer to this algebra question.

Starting at home, emily traveled uphill to the hardware store for 606060 minutes at just 666 mph. she then traveled back home along the same path downhill at a speed of 121212 mph. what is her average speed for the entire trip from home to the hardware store and back?

If a company produces x units of an item at a cost of $35 apiece the function c=35x is a of the total production costs