There's a slick trick when two numbers share the same tens digit and their units add to 10 (like 84 × 86). What is it?

Multiply the shared leading part by one more than itself, then append the product of the units.

For t a × t b with a + b = 10: the answer is t·(t+1) followed by a·b (two digits):

• 84 × 86 → 8×9 = 72, and 4×6 = 24 → 7224.
• 43 × 47 → 4×5 = 20, and 3×7 = 21 → 2021.

Check: 84 × 86 = 7224. ✓ It's (10t+a)(10t+b) = 100·t(t+1) + ab when a+b = 10. Squaring a number ending in 5 is the special case a = b = 5.