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This is a easy difficulty Statistics & Math question, commonly asked during Onsite rounds at Meta.

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This question is commonly asked for Data Scientist candidates at Meta during technical interviews.

Determine Probability of Friend Request Being Fake

Last updated: Mar 29, 2026

Quick Overview

This question evaluates understanding of conditional probability, base-rate reasoning, and probabilistic modeling of differing event rates. It is commonly asked to assess whether an applicant can integrate prior class proportions with unequal per-account event rates when estimating source probabilities, belongs to the Statistics & Math category, and tests conceptual understanding of probability rather than implementation-level skills.

Scenario

You operate a platform where 95% of accounts are real and 5% are fake. Fake accounts send 10 times as many friend requests per account as real accounts.

Question

If you receive a random incoming friend request, what is the probability that it is from a fake account?

Assumptions

"10× more" means each fake account sends friend requests at a per-account rate 10 times that of a real account.
You are equally likely to receive any given friend request (ignoring network structure or targeting effects).

Hint

Apply Bayes’ theorem: posterior ∝ prior × likelihood.

Solution

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