For example for this problem:
I used the info in the question to come up with a system of equations:
This system looks reasonably easy to manipulate to find the regular, weekday price. But surely you can't treat quantities of a specific type (e.g. height, weight, price, etc.) as interchangeable with each other in the same way that you would in a problem involving only ordinary numbers?
The answer is apparently 20, but I am not sure how you can get to this solution algebraically.