PracHub
QuestionsCoachesLearningGuidesInterview Prep
|Home/Statistics & Math/Amazon

Determine Probability of Both Children Being Boys

Last updated: Jun 15, 2026

Quick Overview

Evaluates conditional probability reasoning in the classic two-children problem with different observation mechanisms. Strong answers enumerate ordered outcomes, condition correctly, and compare at-least-one, older-child, and random-child cases.

  • medium
  • Amazon
  • Statistics & Math
  • Data Scientist

Determine Probability of Both Children Being Boys

Company: Amazon

Role: Data Scientist

Category: Statistics & Math

Difficulty: medium

Interview Round: Technical Screen

##### Scenario A classic conditional-probability brain-teaser used in data science screens to test probabilistic reasoning and the discipline of stating assumptions before computing. ##### Question A family has two children. You learn that at least one of them is a boy. 1. What is the probability that both children are boys? Explain your reasoning and state your assumptions clearly. 2. How does the answer change if instead you are told that the older (first) child is a boy? 3. How does the answer change if a child is selected at random from the family and that child turns out to be a boy? ##### Hints Enumerate the equally likely ordered outcomes BB, BG, GB, GG, then condition on the given information by removing the outcomes it rules out. Pay close attention to *how* the information was obtained — different selection mechanisms produce different answers.

Quick Answer: Evaluates conditional probability reasoning in the classic two-children problem with different observation mechanisms. Strong answers enumerate ordered outcomes, condition correctly, and compare at-least-one, older-child, and random-child cases.

Related Interview Questions

  • Compute an A/B test p-value by hand - Amazon (medium)
  • Compute and interpret quantile loss vs RMSE - Amazon (medium)
  • Compute CIs, power, and multiple testing - Amazon (medium)
  • Plan and analyze an A/B test - Amazon (hard)
  • Compute p-values, CIs, and adjust multiples - Amazon (Medium)
|Home/Statistics & Math/Amazon

Determine Probability of Both Children Being Boys

Amazon logo
Amazon
Jul 12, 2025, 6:59 PM
mediumData ScientistTechnical ScreenStatistics & Math
21
0

Conditional Probability: Two Children

A family has two children. You learn that at least one of them is a boy.

Answer the questions below and state your assumptions clearly.

Constraints & Assumptions

  • Assume each child is independently a boy or girl with probability 1/2 unless you state otherwise.
  • Treat birth order as relevant when enumerating equally likely outcomes.
  • Pay attention to how the information is obtained.
  • Distinguish "at least one is a boy" from "the older child is a boy" and from "a randomly selected child is a boy."

Clarifying Questions to Ask

  • Are boys and girls equally likely and independent?
  • Is the family sampled uniformly from all two-child families?
  • How did we learn that at least one child is a boy?
  • Are we conditioning on a statement about the family or on observing a randomly selected child?

Part 1 - At Least One Boy

Given only that at least one child is a boy, what is the probability that both children are boys?

What This Part Should Cover

  • Enumerate ordered outcomes BB, BG, GB, and GG.
  • Condition by removing GG.
  • Compute the probability of BB among the remaining equally likely outcomes.

Part 2 - Older Child Is a Boy

How does the answer change if you are told that the older child is a boy?

What This Part Should Cover

  • Condition on outcomes where the first child is B.
  • Compare BB and BG.
  • Explain why this conditioning information is more specific.

Part 3 - Randomly Selected Child Is a Boy

How does the answer change if a child is selected at random from the family and that child turns out to be a boy?

What This Part Should Cover

  • Condition on the observation process, not only the family composition.
  • Weight BB, BG, and GB by the probability that a randomly selected child is a boy.
  • Explain why the answer differs from the generic "at least one boy" statement.

Follow-up Questions

  • What if the statement is "at least one child is a boy born on Tuesday"?
  • How would the answer change if boys and girls were not equally likely?
  • Why do different observation mechanisms produce different answers?
Loading comments...

Browse More Questions

More Statistics & Math•More Amazon•More Data Scientist•Amazon Data Scientist•Amazon Statistics & Math•Data Scientist Statistics & Math

Write your answer

Your first approved answer each day earns 20 XP.

Sign in to write your answer.
PracHub

Master your tech interviews with 8,000+ real questions from top companies.

Product

  • Questions
  • Learning Tracks
  • Interview Guides
  • Resources
  • Premium
  • For Universities
  • Student Access

Browse

  • By Company
  • By Role
  • By Category
  • Topic Hubs
  • SQL Questions
  • AI Coding Questions
  • Compare Platforms
  • Discord Community

Support

  • support@prachub.com
  • (916) 541-4762

Legal

  • Privacy Policy
  • Terms of Service
  • About Us

© 2026 PracHub. All rights reserved.