Here's a mental maths trick I really like.
Want to calculate 75² without pen and paper?
Take the first digit (7), multiply it by the next number (8), and put 25 at the end.
7 × 8 = 56
So, 75² = 5625.
It also works for:
35² = 1225
95² = 9025
And even 115² = 13225.
Does anyone know a simple way to explain why this works, especially to school students?
I'd also love to hear other shortcuts that are useful for teaching maths.