2016 AMC 8 Problem 24

Attempt Problem 24 of the 2016 AMC 8 below, then check your answer against the video solution and professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2016 AMC 8 solutions, or check the answer key.

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

24.

The digits 1,1, 2,2, 3,3, 4,4, and 55 are each used once to write a five-digit number PQRST.PQRST. The three-digit number PQRPQR is divisible by 4,4, the three-digit number QRSQRS is divisible by 5,5, and the three-digit number RSTRST is divisible by 3.3. What is P?P?
1 1
2 2
3 3
4 4
5 5
Answer: A
Concepts:divisibilitycasework
Small hint:
The divisibility by 55 condition determines SS.
Big hint:
Once SS is known, list the possible two-digit endings QRQR that make PQRPQR divisible by 44.
Video solution:
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Written solution:
Since QRSQRS is divisible by 5,5, we know that S=5.S = 5.
Since PQRPQR is divisible by 4,4, QRQR equals either 12,12, 24,24, or 32.32.
If QR=12,QR=12, then R=2,R=2, so RST=25TRST=25T cannot be divisible by 33 using the remaining digits. If QR=32,QR=32, then R=2R=2 again, giving the same obstacle. Thus QR=24,QR=24, and then RST=45T.RST=45T. Among the remaining digits, only T=3T=3 makes 453453 divisible by 33.
Therefore, PQRST=12453.PQRST = 12453.
Thus, A is the correct answer.
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