Square a Number Ending in 5
Square any number ending in 5 instantly.
(a5)² → a×(a+1), then write 25
1How it works
For a number ending in 5, multiply the part before the 5 by the next whole number, then simply attach 25 at the end.
2How to apply (in seconds)
- Drop the 5 and take the remaining number a.
- Multiply a by (a + 1).
- Write 25 after that result.
3Worked examples
Example 1 Worked solution
Q. Find 65².
- a = 6, so a×(a+1) = 6×7 = 42.
- Attach 25.
✓ 4225
Example 2 Worked solution
Q. Find 115².
- a = 11, so 11×12 = 132.
- Attach 25.
✓ 13225
4When to use it
- ✦Squaring 2- and 3-digit numbers ending in 5.
- ✦Speeding up area and mensuration sums.
5Cautions & tips
- !Works for any length of number before the 5.