Subtract a 5% to the total if a field has more than 20 products

Hi, im working in a Wcart and im trying to achieve this: when a text-input named “cantidad” has a value equal or more than 10, the text-input “total” with the sum of all prices as value, should subtract a 5% of the “total” value.
I think this should be done with ternary operation, but im not very good with it yet.
I’ll apreciate any help. Thnks in advance