Gst concession for disabled on purchase of car

Hey friends

Actually this is quite urgent  I am planning to buy a car (ignis zeta). I came to know that I get an exemption in GST on purchase of car. Which means that I am charged @ 18% GST as against 28% for normal person. I applied to the Ministry of Heavy Industries  Delhi and they issued me a certificate asking the dealer to charge GST @ 18%. However they have no idea as to how the calculation is to be done. The ex showroom price of the car is Rs. 631393/-.. Kindly guide me with example regarding the method of calculation considering the ex showroom price above.. Thanks in anticipation