Composite Numbers Math Example 1

Solution

1. 1
   Test divisibility by primes up to $\sqrt{91} \approx 9.5$: primes to test are $2, 3, 5, 7$.
2. 2
   $91$ is odd (not divisible by $2$). Digit sum $= 10$ (not divisible by $3$). Last digit $\neq 0, 5$ (not by $5$).
3. 3
   Test $7$: $91 \div 7 = 13$. Yes! $91 = 7 \times 13$.
4. 4
   $91$ is composite with factor pair $(7, 13)$.