Numbers, Classification and the Odd-Even Rules

04 Try it yourself Interactive

Classify any number in as few divisions as possible

Choose a number and step through it. The console checks the parity, works out the square-root limit, then tries primes in order and halts the instant one divides. Watch the divisions counter — that number is the payoff for knowing where to stop.

91 — the classic false prime

7 × 13

The composite choices here are the ones placement papers actually use, because each one looks prime at a glance.

Press Next to work through it one step at a time.

Parity–

Test up to–

Divisions–

Verdict–

step 0 / 0