Check Box Formula

[alopecito]

Hi, I'm doing an Invoice and I would like to know if this is possible, I need to add a check box, if the box is checked I need to reduce the total amount by 10%....If the box is leave blank the total shouldn’t be modified.
I guess I Need an “IF” formula but I spend a lot of time researching it and I found nothing...I appreciate any help....THANKS

I attached the example file.

[Tommy]

Hi alopecito,

I changed the checkbox to data validation. Then adjusted the formula. Please see attached.

[alopecito]

Thanks!!!