2017 AIME II Problem 4

Attempt Problem 4 of the 2017 AIME II 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 2017 AIME II solutions, or check the answer key.

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

4.

Find the number of positive integers less than or equal to 20172017 whose base-three representation contains no digit equal to 0.0.

Answer: 222
Concepts:number basedigitscasework
Difficulty rating: 2230
Solution:

A positive integer has no 00 in base three exactly when every digit is 11 or 2.2. For k=1,2,,6k = 1, 2, \ldots, 6 there are 2k2^k such kk-digit numbers, and all of them are at most 2222223=728<2017.222222_3 = 728 \lt 2017.

Since 2017=22022013,2017 = 2202201_3, a seven-digit string of 11s and 22s is at most 20172017 exactly when it begins with 11,11, 12,12, or 21:21: any string beginning 2222 already beats 220220132202201_3 at the third digit, since its digits are nonzero. That gives 325=963 \cdot 2^5 = 96 seven-digit numbers.

The total is 2+4+8+16+32+642 + 4 + 8 + 16 + 32 + 64 +96=222.+ 96 = 222.

← Problem 3#3
Full Exam

Problem 4 in Other Years