To square a number ending in 5:

Step 1: Take the number formed by the digits in front of the 5. Multiply this by the next whole number.

Step 2: To that product, affix the number 25.

Examples:

How much is 35²?
3 x 4 = 12
Write down 12 and affix 25.
Answer is 1225.

How much is 105²?
10 x 11 = 110
Write down 110 and affix 25.
Answer is 11025.