In order to answer, I would first need to know Jana's speed, not that she biked at a constant rate.
Each dvd cost $7.50 how? Your friends buy 8 dvd's and in total is 60, you want to divide 60 from 8 and find that it's 7.50 and then multiply 8 to 7.50 and it should be 60. Hope this helps!!
125 divided by 100 is 1.25, 1.25 times 15 is 18.75.She tips $18.75