If you have a calculator:
1) To find 1%, divide by 100 (70 / 100 = 0.7)
2) Then, multiply by the % you need (0.7 x 13 = 9.1)
Therefore 70 - 9.1 = 60.9
If you don't have a calculator:
1) Find 50%, 10%, 5%, and 1%. With these, you can make any % you need.
50% = X / 2 (= 35)
10% = X / 10 (= 7)
5% = first, do 10%, then divide that by 2 (= 3.5)
1% = X / 100 (= 0.7)
Where X is the number you want to work out % for
2) Add those % up to get the amount you want
10% + 1% + 1% +1% = 13%
7 + 0.7 + 0.7 + 0.7 = 9.1
Therefore 70 - 9.1 = 60.9
Hope that helps :)