Sarah,
Let x = cost of notebook
let y = cost of thumbdrive
For Troy 5x + 5y = 75
For Lisa 2x + 6y = 78
Troy's equation let's divide it by 5 and get
x+ y = 15 multiply by -2
-2x -2y = -30
2x + 6y = 78 Add equations
4y = 48
Solve for y. Then plug it in one of the original equations and solve for x. Check by plugging into the other original equations
Step-by-step explanation: