let x = cost of cam, let y = cost of case
x + y = 310
x = 300 + y
y = 310 - x
Replace the y in the second equation with (310 - x) to get:
x = 300 + 310 - x
x = 610 - x
2x = 610
x = 305
so the camera costs $305. And 310 - 305 = 5, so the case is $5.
100 - 5 = $95 in change.