Corbettmaths

Building blocks:

50%     divide by 2

25%     divide by 4 (half and half again)

10%     divide by 10

1%     divide by 100
Other percentages:

2%     get 1%, then double it (or add 1% and 1%)

20%     get 10%, then double it (or add 10% and 10%)

5%     get 10%, then half it

15%     get 10%, get 5% (by halving 10%) then add the 10% and the 5%

30%     get 10% and times by 3 (or add 10%, 10% and 10%)

33 1/3%     divide by 3

75%     get 50% and 25% and add them

40%     get 10% and times by 4

90%     get 10% and take it away from 100% (100% is the number you start with)

95%     get 5% (see above) and take it away from 100% (100% is the number you start with)

110%     add 10% to 100% (100% is the number you start with)

101%     add 1% to 100% (100% is the number you start with)

150%     add 50% to 100% (100% is the number you start with)

250%     add 100% (start number), 100% (start number) and 50%