Counting numbers up to 100000 with digit sum nine
Question
How many positive integers not exceeding have decimal digits adding up to ? (Examples: , , .)
Short answer
. itself has digit sum 1, so pad every smaller number with leading zeros to five digits. We need non-negative solutions of ; no digit can exceed 9, so stars and bars gives .
Adapted from QuantQuestions.io; question and answer rewritten by SelfMadeQuant.com.
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Open the worked solutionLessons behind this question
- Counting and Combinatorics
- Sample Spaces, Events, and the Probability Axioms
- Conditional Probability, Total Probability, and Bayes' Rule
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