The quadratic formula and what the discriminant tells you before you finish, plus why exponents, roots and logarithms are three views of one relationship.
The divide-multiply-subtract-bring-down cycle worked through on real numbers, including two-digit divisors, decimals, and the check that catches every slip.
Decimal places, significant figures, and the two competing half-way rules — plus why rounding 2.675 to two decimals legitimately gives 2.67.
Simplifying, scaling, splitting a total three ways, and the ratio-versus-fraction mix-up that causes most wrong answers — with worked examples.
One word, three different sums. How to find a percentage of a number, work out percentage change, and reverse a discount — with the formula for each.
A four-function calculator does exactly what you type — which is the problem. Here's how order of operations, the percent key, and memory really behave.