How to Report income in this scenario

Hello i am new to this forum, 

i have a query , i do retailing business and my annual business turnover is 20lakhs and i want to file itr under presumptive taxation next year.

so if my business total turnover is 20lakhs , out of these 20lakhs i get 8lakhs through bank transaction (online) and 12lakhs cash , and i have a profit margin of 40% , it means my net income is 8lakhs. So the rest 12 lakhs are the payment i have to make to my supplier for the goods he supplied to me on which i made my net income of 8lakhs, and i paid him this 12lakhs through cash because of ease.

And i will report my actual net profit which is 8lakhs but the main question is what should i fill in below columns which it dept asks:

1)Mention how much turnover you recevied through online transactions, 

2)Mention how much turnover you recevied through cash transactions,

3)Mention income earned from online transactions ,

4)Mention income earned from cash transactions 

i know the answer of first two that i received 8lakhs online and 12lakhs through cash, but what to fill in 3rd and 4th , because my income from cash is zero because i spent all 12lakhs in payment to supplier so my net profit is zero for cash transactions , while for online transactions i have all 8lakhs lying in my bank account , so there net profit margin is coming 100%

what to do in this scenario, please guide....

(i know its a long post , sorry for the same)