Quantitative Aptitude :: Permutation and Combination - Discussion

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Explanation:
He can pay by all 10 rupee notes in 5 days = 1 way
3 Ten rupee + 1 twenty rupee = 4!/(3!*1!) = 4 ways
1 Ten rupee + 2 twenty rupee notes = 3!/(2!*1!) = 3 ways
Total ways = 1 + 4 + 3 = 8