Query with special condition like 2%2 =0

When user buy two of the item will get a discount. Now the problem is when the user buy four qty of the product i cant match it out. Therefore i need to do some math in query. Like quantity%database.quantity == 0.

How can i do that in my query

        $promotionID = Promotion::where('product_id',$product)
			->whereRaw('promotion_quantity%'.$quantity.'=0')
                            ->get();