calculation code for quantity of products by ss

Dear Sir,

   In my store, products have different quantites such as unit,box,display..Please see this example I explained below(what i want).

Please see this link: http://myakr-corporation.myshopify.com/collections/frontpage-alpha-asc/products/3musk (Storefront password: chieyu)

Here if i add quantity count between 26-124, in add to cart page the quantity should be multiple of 25.

Brief explanation:        

BOX                              QTY (want to display qty as)                              PRICE

26                                    50                                                                 26*26.20

51                                    75                                                                 51*26.20 and so on upto 124box then

126                                  250                                                                 126*40

How to do in calculation: (26/25)*25=1.04*25

If the value is float(1.04) we will take as 2(as whole number)

So i want to add code in liquid file, I don't know how to add and where is the exact place in liquid file.

Is there an easy fix?