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← JEE Main 2022 · 28 Jul · Shift 1
MathematicsInteger+4 / -1Question 21 of 90

JEE Main 2022 · 28 Jul · Shift 1, Mathematics question 21

Let S be the set of all passwords which are six to eight characters long, where each character is either an alphabet from {A, B, C, D, E} or a number from {1, 2, 3, 4, 5} with the repetition of characters allowed. If the number of passwords in S whose at least one character is a number from {1, 2, 3, 4, 5} is α×56\alpha\times5^6, then α\alpha is equal to ______.

Answer and worked solution

Official answer: 7073

Complement. Passwords of length n with at least one digit: 10n−5n10^n-5^n.

Sum over n = 6, 7, 8, divided by 565^6: (106−56)+(107−57)+(108−58)56=7073\frac{(10^6-5^6)+(10^7-5^7)+(10^8-5^8)}{5^6}=7073.

Answer: 7073