Square Numbers Ending in 5

Problem

Squaring numbers ending in 5 using traditional multiplication is slow.

Solution

Take the first digit(s), multiply by (itself + 1), then append 25.

Benefit

Instant calculation without long multiplication, saves 90% of time.

Example

25² = 2×(2+1) = 2×3 = 6, append 25 → 625

75² = 7×(7+1) = 7×8 = 56, append 25 → 5625

105² = 10×(10+1) = 10×11 = 110, append 25 → 11025