She will be 75 minutes late because you take the 15 minutes per stop and multiply it by the five stops.
Here is the equation:

⇒

You need to get "x" by itself so minus 3 from each side

⇒

⇒

Divide 8 by 2

÷

x=4
So each 4 is equivalent to 4
Lets check:

⇒

⇒

So that means that Marcus practiced the piano for 4 hours and Donnel practiced for 7
1.80 I think. It's actually 1.7995 but in money we round so it's 1.80