How do I calculate price reduction?

I would say y% is 100-x%

And the new value is y% divided by 100 and then times the original value.

Divide the y% value by 100, since % is per 100, as 1% is 1 part of 100, 10% is 10 parts of 100.

As an example, the first column:

100-10% = 90% = y%

90% divided by 100 = 0.9

0.9 times 65 € = 58.50 €