Square Numbers Ending in 5 Instantly

Problem

Squaring numbers like 35, 65, or 85 requires long multiplication that is slow and error-prone.

Solution

Take the tens digit, multiply it by (itself + 1), then append 25 at the end. Done.

Benefit

Square any number ending in 5 in under 3 seconds mentally — no pen, paper, or calculator needed.

Example

25² → 2 × 3 = 6, append 25 → 625 ✓
35² → 3 × 4 = 12, append 25 → 1225 ✓
45² → 4 × 5 = 20, append 25 → 2025 ✓
65² → 6 × 7 = 42, append 25 → 4225 ✓
75² → 7 × 8 = 56, append 25 → 5625 ✓
85² → 8 × 9 = 72, append 25 → 7225 ✓
95² → 9 × 10 = 90, append 25 → 9025 ✓