2004 AMC 10B Problem 2

Attempt Problem 2 of the 2004 AMC 10B below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2004 AMC 10B solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

2.

How many two-digit positive integers have at least one 77 as a digit?
1010
1818
1919
2020
3030
Answer: B
Concepts:digitsinclusion-exclusion
Small hint:
Count the numbers with 77 in the tens place and those with 77 in the units place separately
Big hint:
The number 7777 belongs to both groups
Solution:
The numbers 7070 through 7979 give 1010 with a 77 in the tens place.
The numbers 17,27,…,9717, 27, \ldots, 97 give 99 with a 77 in the units place.
Since 7777 is counted twice, the total is 10+9−1=18.10 + 9 - 1 = 18.
Thus, the correct answer is B.
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