MENTALMATH

Mental math is a toolbox of shortcuts that turn slow paper calculations into fast head math. None of it is magic — every trick is ordinary arithmetic rearranged to be easier to hold in your mind (add the big digits first, round to friendly numbers, split a hard multiply into easy pieces).

The point isn't to replace a calculator; it's to build number sense — to estimate a bill, sanity- check a figure, or square 45 before someone finds their phone. Because the answers are timed and verifiable, these techniques are perfect for Dogfight duels.

Every method in this module is stated with a worked example, its domain of use, and the pitfalls (several popular "tricks" are subtly mis-stated online — the ×11 rule forgets to carry, "left-to-right never carries" is only true sometimes — and those traps are flagged).

Six missions, ordered so each builds on the last:

1. Addition & subtraction — left-to-right, rounding-and-adjusting, balancing.
2. Multiplication tricks — ×11, ×5/25/50, ×9/99, doubling-halving, the cross method, near-100.
3. Squaring shortcuts — ending in 5, near 50, near 100, the (a±b)² identity.
4. Divisibility rules — 2–11, with the reason each works.
5. Percentages & estimation — 10%/1%/5%, tips, the x%·y swap, rounding and Fermi estimates.
6. Checking & Vedic — casting out nines/elevens, and the (correctly de-mythed) Vedic system.