Write the integers from $20$ to $40$ inclusive. Use divisibility by $2,3,5,7$ to identify all

Question

Write the integers from $20$ to $40$ inclusive. Use divisibility by $2,3,5,7$ to identify all the primes in this interval. Explain why no larger prime divisor needs to be tested.

Accepted Answer

$23,\ 29,\ 31,\ 37$. For numbers at most $40$, any composite number has a prime factor no larger than $\sqrt{40}<7$. Testing $2,3,5$ already suffices; including $7$ is harmless.

Primes, HCF and LCM

Primes, HCF and LCM

Solution:

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