Excel Data Validation - Increments of 20 Only Input

I have a worksheet, I only want to allow the user to input a number value that is increments of 20 only (e.g. 20, 40, 60, 80 etc). Ideally, if the user inputs 15 for example, this would automatically round UP to the nearest 20 and display the message (only increments of 20 are allowed). Is this possible with data validation custom formula? Or can I achieve this another way? Any info or a point to the answer would be greatly appreciated.

You could force the user to enter increments of 20 by using a data validation formula something like =MOD(CellRefHere,20)=0

If you need the value to automatically round up to the nearest 20 then you'll want to use a worksheet change event that is fired when any of a range of specified cells is changed and then checking the value and amending if necessary.

BSB