Determining If Numbers Are Evenly Divisible

In my order program, I need to determine if the quantity ordered is full boxes.For example - all of our parts/products have different box quantities, and when a customer orders a certain quantity, I want it to do nothing when I type in the quantity IF the order quantity is even boxes, If it is not even boxes, I want a pop up form with three radio buttons to come up. The three radio buttons will have the next higher even box quantity, the next lower box quantity, and to keep the current quantity and apply a broken box charge. I have this pop-up form done except for determining if the order quantity is an even number of boxes, and then determining the next higher and next lower.

Just to clarify - let's say a customer orders a part that has 60 pieces per box. If he orders 180 pieces, nothing happens and you can continue to put the order on. If he orders 200 pieces, a pop-up form with 3 selections - "keep current quantity and apply broken box charge", or "Change Quantity to 240", or "Change Quantity to 180" comes up.