AMC 10 Step by Step
2022 AMC 10AProblem 24P21-25~12 minPrint

How many strings of length 55 formed from the digits 00 , 11 , 22 , 33 , 44 are there such that for each j{1,2,3,4}j \in \{1,2,3,4\} , at least jj of the digits are less than jj ? (For example, 0221402214 satisfies this condition because it contains at least 11 digit less than 11 , at least 22 digits less than 22 , at least 33 digits less than 33 , and at least 44 digits less than 44 . The string 2340423404 does not satisfy the condition because it does not contain at least 22 digits less than 22 .)

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