75 x 75 = 5625

Method for squaring a number ending in 5:
- take the digit(s) preceding the five (7)
- multiply this number by itself plus one (7×8=56)
- attach 25 to the end (5625)

Mathematical explanation:
(10x + 5)² = (10x + 5)(10x + 5)
(10x + 5)² = 100x² + 100x + 25
(10x + 5)² = 100x(x + 1) + 25