Microsoft interview question (percentage calculation) [on hold]

I am new to this group. I have an Microsoft interview question and seeking your help at the earliest

Question here as follows:

“I buy 100 donuts from Shop A , spending 10 Rupees per each – so total expense is 1000 Rupees”

“I see another Shop B and I found some offers such a way that I buy 60 donuts for 8 Rupees -> (60 donuts x 8 Rupees = 480 Rupees) and (40 donuts x 5 rupees = 200 Rupees) . So total expense in shop B is 680 Rupees”

Now I calculate the profit from Shop B to Shop A as 1 – 680/1000 is almost 32%

How much percentage of profit contributed by only 40 donuts (40 x 5 rupees cheaper) from the total savings of 32%?