Let's assign some random values to it, and say 1 unit of petroleum costs 100 bitcoins, because we all know bitcoins are the best currency :P
If the price goes up by 10%, it is now 100*1.1 = 110bitcoins for 1 unit.
However he's only getting 0.9 units now, so multiplying 110 by 0.9 gives us 99 bitcoins. His petroleum bill used to be 100 (which is why I picked 100 as it's easier), and now it is 99. Therefore his petroleum bill has reduced by 1%
