🎓 Deeper Dive — Adults

Subtraction is the inverse of addition: a − b is the unique x such that b + x = a.

It is not commutative (5−3 ≠ 3−5) and not associative. Negative numbers (introduced by Indian mathematician Brahmagupta, 7th century) closed the natural numbers under subtraction → integers ℤ.

Mental shortcut for borrowing: round up the subtrahend, then add the difference back. 84 − 27 → 84 − 30 + 3 = 57.