x = chips purchased
y = pretzels purchased
x + y = 23
0.25x + 0.50y = 7.75
You can use substitution or elimination for this one. I will use substitution.
x = 23 - y
0.25 (23 - y) + 0.50y = 7.75
5.75 - 0.25y + 0.50y = 7.75
5.75 + 0.25y = 7.75
0.25y = 2
y = 8
Now we will plug the y value in to one of the starting equations.
x + y = 23
x + 8 = 23
x = 15
Helen bought 15 chips and 8 pretzels.