Skip to content

Higher or lower on a d100 paid the second value

Question

A fair die with faces 11 to 100100 is rolled and you see the result xx. You then predict whether a second roll yy will be at least xx ("higher") or strictly below xx ("lower"). If your prediction is right you are paid yy dollars, otherwise nothing. Playing to maximise expected payout, what is the smallest first roll xx for which you should predict "lower"?

Short answer

7272. Predicting lower pays 1100∑y<xy=x(x−1)200\tfrac{1}{100}\sum_{y<x}y=\tfrac{x(x-1)}{200} on average, while predicting higher pays 1100∑y≥xy=(100+x)(101−x)200\tfrac{1}{100}\sum_{y\ge x}y=\tfrac{(100+x)(101-x)}{200}. Lower wins when x(x−1)>(100+x)(101−x)x(x-1)>(100+x)(101-x), which first happens at x=72x=72 (the crossover is near 71.671.6): at 7171, 4970<51304970<5130, while at 7272, 5112>49885112>4988.

Adapted from QuantQuestions.io; question and answer rewritten by SelfMadeQuant.com.

See the hint and the full worked solution in the question bank (free account).

Open the worked solution

Lessons behind this question

Start with a free lesson: Expectation, Variance and Covariance is part of a free topic. Create a free account, no card needed, to read it in full with worked solutions.

Read it free