Math Tips

Multiplying with 11 past 10:

Write the last and first number, and add the two to fill the middle. The number will be 3 digits.

76 × 11 = ?

7_6

7 + 6 = 13

836

For numbers in the hundreds, it is 4 digits. Do the same but instead of adding the numbers ar the end for the middle, add the middle number with each of the edges.

253 × 11 = ?

2__3

2 + 5 = 7

27_3

5 + 3 = 8

2783

Note: if you ever get a number in the 10s when adding, move the 1 to the first number, similar to in regular addition.

Square roots of 2 digit numbers ending in 5:

Multiply the number in the tens with the number 1 higher than it, and put 25 at the end.

45 × 45 = ?

4 × ( 4 + 1 )

4 × 5 = 20

2025

Multiplying 2 digit numbers with the same first

Multiplying with 9:

Save the second digit, multiply the first digit with 9 and subtract the second digit from the tens of it. Then add 180 x second digit.

9 × 6 = 54

5 - 7 = -2

734