Find the odd one out:

Correct — A.
Rule: read # as division; of +, −, × and ÷, it is the operation that makes three statements true.

20 ÷ 4 = 5 ✓, 25 ÷ 5 = 5 ✓, 15 ÷ 3 = 5 ✓
10 ÷ 5 = 2, not 5 ✗

10 # 5 = 5 holds only if # is subtraction, and subtraction fails the rest (20 − 4 = 16) → option (a).

Why the others are wrong

• (b)
  20 # 4 = 5 — 20 ÷ 4 = 5 is true, so this statement follows the division rule with the majority and cannot be the odd one out.
• (c)
  25 # 5 = 5 — 25 ÷ 5 = 5 holds under the same division rule as 20 # 4 and 15 # 3, so it belongs to the group.
• (d)
  15 # 3 = 5 — 15 ÷ 3 = 5 fits the division rule too. It is one of the three statements that agree on ÷, so it is not the odd one.

A symbol odd-one-out asks for the operation that makes most of the statements true. Try each basic operation on all four statements until one fits three of them; the statement it fails is the answer.

Here division fits three. Subtraction fits 10 # 5 = 5 alone, which is why that statement stands apart.

Key facts

• Under ÷: 20 ÷ 4, 25 ÷ 5 and 15 ÷ 3 all give 5, while 10 ÷ 5 gives 2.
• Under −: 10 − 5 = 5 holds, but 20 − 4 = 16, 25 − 5 = 20 and 15 − 3 = 12.
• Every statement ends in = 5, so the result alone separates nothing.

Common traps

• Taking 10 # 5 = 5 as the regular one because 10 − 5 = 5 is the first rule that comes to mind
• Stopping when an operation fits one statement, instead of finding the one that fits three

No directly related past PYQ was found.