Shortcuts

Question 1

Roman Numeral

I

Answer 1

1

Question 2

Roman Numeral

V

Answer 2

5

Question 3

Roman Numeral

X

Answer 3

10

Question 4

Roman Numeral

L

Answer 4

50

Question 5

Roman Numeral

C

Answer 5

100

Question 6

Roman Numeral

D

Answer 6

500

Question 7

Roman Numeral

M

Answer 7

1000

Question 8

÷ 2

Answer 8

Half

Question 9

Remainder when ÷ 2

Answer 9

Even = 0
Odd = 1

Question 10

÷ 3

Answer 10

If the product of all the digits is divisible by 3, the whole number is

Question 11

Remainder when ÷ 3

Answer 11

Remainder of the sum of the digits ÷ 3

Question 12

÷ 4

Answer 12

Cut in half, cut in half again

Question 13

Remainder when ÷ 4

Answer 13

Find the remainder of the last two digits.

Question 14

÷ 5

Answer 14

Double all the digits and move the decimal over one place to the left

Question 15

Remainder when ÷ 5

Answer 15

The remainder of the number is the last digit. If the last digit is greater than five, subtract five to get the remainder.

Question 16

÷ 6

Answer 16

The number is even, and the product of all the digits is divisible by 3

Question 17

Remainder when ÷ 6

Answer 17

No known rule

Question 18

Remainder when ÷ 7

Answer 18

Subtracting 2 times the last digit from the rest gives a multiple of 7. (Works because 21 is divisible by 7.)483: 48 − (3 × 2) = 42 = 7 × 6.

Question 19

÷ 8

Answer 19

Cut in half, cut in half, cut in half again

Question 20

Remainder when ÷ 8

Answer 20

Find the remainder of the last three digits.

Question 21

÷ 9

Answer 21

If the product of all the digits is divisible by 9, the whole number is

Question 22

Remainder when ÷ 9

Answer 22

Find the remainder of the sum of the digits ÷ 9

Question 23

÷ 10

Answer 23

Ends in a 0

Question 24

÷ 11

Answer 24

if the difference of the alternating sum of digits of the number is a multiple of 11 (e.g. 2343 is divisible by 11 because 2 - 3 + 4 - 3 = 0, which is a multiple of 11);

Question 25

÷ 12

Answer 25

The sum of the digits is divisible by 3 & 4