A salesman is appointed on the basic salary of Rs. 4,000 per month on the condition that every sale of Rs. 20,000 he will get 60% of basic salary and x% of the sales as a commission. This scheme does not operate for the first Rs. 20,000. If the total sale is Rs. 80,000 and he wants to earn 17,200 in a particular month, then find the value of x.