220 is divided by 11 (not 10) because the gst is already included in the total amount
to get the gst to an amount that doesn't have it you divide by 10 and then add that to the original
to get the original price of an amount that already has gst included you divide it by 11 and then times that by 10
so therefore to just get the gst for the $242 you divide by 11 (which is 22)
and then to get the gst for the $100 you divide by 10 (which is 10)
and then add those two together
so the total gst is $32, which is c