##### Scenario Product manager asks a quick probability check during a design review to gauge statistical intuition. ##### Question You flip a fair coin repeatedly until the sequence Heads–Tails (HT) appears consecutively. What is the expected number of flips required? ##### Hints Model as states: start, last flip H, and absorbing HT; use expected value equations.

This question evaluates a candidate's proficiency in probability theory and stochastic-process reasoning, including Markov-style state modeling and expected-value analysis for stopping times.

How do I approach Statistics & Math interview questions?

Statistics & Math questions require understanding of core concepts and practice. PracHub provides solutions with explanations to help you master statistics & math interviews.

What difficulty level is this interview question?

This is a easy difficulty Statistics & Math question, commonly asked during Technical Screen rounds at Yahoo.

What role is this question designed for?

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

Calculate Expected Flips for Heads-Tails Sequence

Yahoo

Aug 4, 2025, 10:55 AM

Data Scientist

Technical Screen

Statistics & Math

0

Probability Check — Waiting Time for HT

Setup

You repeatedly flip a fair coin (independent flips; P(H)=P(T)=1/2).
Stop as soon as the consecutive pattern HT appears for the first time.

Question

What is the expected total number of flips required until HT first appears?

Hint: Model states as Start (no useful last flip), H (last flip was H), and an absorbing state (HT seen). Set up and solve expected-value equations.

Solution

Show

Comments (0)

Sign in to leave a comment

Loading comments...

Browse More Questions

More Statistics & Math•More Yahoo•More Data Scientist•Yahoo Data Scientist•Yahoo Statistics & Math•Data Scientist Statistics & Math

Calculate Expected Flips for Heads-Tails Sequence

Quick Overview