How do you square any number ending in 5?

Multiply the digits before the 5 by the next whole number up, then stick "25" on the end.

For a number n5: compute n × (n + 1), then append 25.

• 25² → 2 × 3 = 6 → 625.
• 35² → 3 × 4 = 12 → 1225.
• 85² → 8 × 9 = 72 → 7225.
• 115² → 11 × 12 = 132 → 13225.

This is the Vedic Ekadhikena Purvena ("by one more than the one before") sutra. It works because (10n+5)² = 100·n(n+1) + 25.