if we don't want to get confused what we can do is 2 equations, one that represents the relationship between the box and the candy, and another that represents the money
x = candy
y = box
x = y + 19
x + y = 20
we replace in the second equation the value of x by (y + 19)
x + y = 20
(y + 19) + y = 20
y +y = 20 - 19
2y = 1
y = 1/2
y = 0.5
we replace in the first equation the value of y by its value
x = y + 19
x = 0.5 + 19
x = 19.5
so the candy value is $ 19.5 and the box value is $ 0.5
