It's easy to explain this using numbers- Let's say you buy something that costs $16. 1/4 of that is $4. 16 - 4 = 12.
12/16 = 6/12 = 3/4
Another way of explain this is using subtraction with fractions:
1 - 1/4 = 1/1 - 1/4 = 4/4 - 1/4 = 3/4
Therefore the customers will pay 3/4 of the original part.
