A prime $p$ is called a Sophie Germain prime when $2p+1$ is also prime. How many of the primes below 20 are of this kind?

Run through the primes below 20 one at a time: 2 → 5 ✓, 3 → 7 ✓, 5 → 11 ✓, 7 → $15=3\times 5$ ✗, 11 → 23 ✓, 13 → 27 ✗, 17 → 35 ✗, 19 → 39 ✗. Four of them work: 2, 3, 5 and 11. The trap 3 drops the very first one, because 2 is the odd man out among the primes — the only even one — and gets overlooked. The trap 8 is simply the number of primes below 20, i.e. every candidate tested, and 6 comes from wrongly passing 7 and 13 without factorising 15 and 27.