How do I make calculations with percentages?

There is a simple way of making calculations with percentages.
Say you want to subtract from a numerical value x,20%. Then multiply it with 80/100.
Say you want to add 20%.Then multiply with 120/100.
Say you want to add 70%. Then multiply with 170/100.
In short,if you want to add x%,then multiply with (100+x)/100. If you want to subtract y% then multiply with (100-y)/100.Note that you can't subtract more than 100%.
Easy,right?
Now we will look on a particular tricky question. We have a TV that costs 1000 pounds.Then,the price of TV is reduced with 20%.After some time,the TV is increased with 20%. What is the final price of a TV?
Don't fall into the trap of saying that the final price is the same with the initial one,therefore 1000 punds.
1.) We reduce with 20% : 100080/100=800 pounds
2.) We increase the current price with 20%,so: 800120/100=960 pounds.
So the final price is 960 pounds.