5. increase £101 by 16%
6. decrease £36 by 21%
7. andy buys a top hat that has been reduces in a sale by 35%. if the sale a price is 13, what was it's original price.

pls and show the method! !

There is a very simple way of dealing with percentages.
"increase" means add
"decrease/discount/reduce" means subtract
Now, we know that the original price is 100%. 
100% = 1 (like 50% = 0.5)
So if an object's price is x and it is 20% off, we get:
100%x - 20%x
Taking x common,
x(100% - 20%)
= 0.8x
Therefore, if we fist add/subtract the percentages we can multiply directly with the original to get the answer. This is as follows:
5] Increase 101 by 16%
1 + 0.16 = 1.16
So the answer is:
101 * 1.16
= 117.16
6] Decrease 36 by 21%
1 - 0.21 = 0.79
The answer is:
36 * 0.79
= 28.44
7] Let original price be x, we know it is 35% off
1 - 0.35
= 0.65
0.65x = 13
x = 20