Pair things up. If everything has a partner, the number is even. If one's left out alone, it's odd. That's the whole rule.
Quick check
You add three odd numbers together. Is the result odd or even?
Your turn
Without computing it, is 1 + 2 + 3 + 4 + … + 99 odd or even?
Exactly half of all whole numbers are even and half are odd — forever. The pattern never breaks.
Recap
- Pair things up: a partner for everyone → even; one left out → odd.
- Last digit 0/2/4/6/8 → even; 1/3/5/7/9 → odd.
- even ± even = even, odd ± odd = even, mixed = odd.
- odd × odd = odd; any product with an even factor is even.
Quick checks
- Ends in 0, 2, 4, 6, 8 → even
- Ends in 1, 3, 5, 7, 9 → odd
- Even + even = even · odd + odd = even · even + odd = odd
- Odd × odd = odd, every other product is even
Half of all whole numbers are even, half are odd. Forever. The pattern never breaks.