Tutor @josgartihmetic sets up the problem in the usual way using two variables:
c+p = 15
4c+2.75p = 50
She then suggests solving the problem using substitution. That is a perfectly good algebraic method; but when the two equations are in this form, elimination is much easier.
Multiply the first equation by 4 and then compare the two equations using subtraction:
4c + 4p = 60
4c + 2.75p = 50
-----------------
1.25p = 10
p = 10/1.25 = 8
ANSWER: 8 bottles of pop and 15-8=7 bags of chips
If a formal algebraic solution is not required, you can get the answer much faster with a little logical reasoning and mental arithmetic -- and using virtually the same calculations. It goes like this:
(1) If all 15 items were bags of chips, the cost would be 15 times $4, or $60. That is $10 more than the actual cost.
(2) Each bottle of pop costs $1.25 less than each bag of chips.
(3) To bring the cost down $10 from $60 to $50, the number of bottles of pop must be $10 divided by $1.25, which is 8.
And again the answer is 8 bottles of pop and 7 bags of chips.