Ordering in Multiples of 20

Summary

Our client has a requirement where few items are supposed to be ordered in Multiples

Content

Hi,

Our customer has a requirement that for few items , the order can have that item in only multiples of 20 , meaning the item can be entered only as 20,40,60 ...so on , anything less than 20 or more than 20 like 19 or 21 should not be allowed 

1. How do we ensure that the item can only be entered in Multiples of 20 

2. How do we restrict or disallow anyone from entering less that 20 or not in multiples of 20