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 Â Â multiply by -2
-2x -2y  = -30
2x + 6y = 78 Â Â Add equations
   4y = 48
Solve for y. Â Then plug it in one of the original equations and solve for x. Â Check by plugging into the other original equations
Step-by-step explanation: