# The Serpent-Tail

After defeating the octangular spirits, you find yourself needing some laundry powder. However, you can't find any at your usual shop, and so you have to ask one of the employees, who happens to be a serpent-tail. You know that serpent-tails have 5 tails, which can understand and answer questions, but only with cracks and slaps. The 5 tails each follow distinct rules, depending on the true answer:

Tail Yes No
A Crack Slap
B Slap Crack
C Random Random
D Crack Crack
E Slap Slap

What is the quickest (fewest questions on average) strategy to find out if the store has laundry powder in stock? You can ask a question to multiple tails at once

• What happens if you ask a question with an unknown answer? Aug 7, 2021 at 14:25
• @bobble They don't answer Aug 7, 2021 at 14:28
• Since the serpent-tail's job is to help customers, rot13( Cyrnfr nafjre jvgu gur penpx-sbe-lrf-naq-fync-sbe-ab gnvy bayl. Qbrf gur fgber unf ynhaqel cbjqre va fgbpx? ) seems like a very effective approach. :-)
– Bass
Aug 7, 2021 at 15:07
• very similar to this question : puzzling.stackexchange.com/questions/110684/… Oct 17, 2021 at 0:25

If I were to ask you if there was laundry powder in stock, would you answer with a crack?

Explanation:

If there is laundry powder, tail A answers "crack" to "is there laundry powder?" and "crack" to "would your answer be a crack?"; tail B answers "slap" and "crack". Thus either way, there are at least 3 answers of "crack" (counting tail D, not caring about the random). So if there are >= 3 cracks, the answer is yes. With similar logic >= 3 snaps means no.

2 questions.

First question:

Are you a tail?

Now we will figure out who to ask the next question to:

The answer to Q1 is "yes". Two of the deterministic tails will slap and two will crack. The random tail will join one of those groups. Simply choose the two tails with the same response. They are guaranteed to not have a random tail in their group.