1961 AMC 12 Problem 17

Attempt Problem 17 of the 1961 AMC 12 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 1961 AMC 12 solutions, or check the answer key.

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

17.

In the base ten number system the number 526526 means 5⋅102+2⋅10+6.5\cdot10^2+2\cdot10+6. In the Land of Mathesis, however, numbers are written in the base r.r. Jones purchases an automobile there for 440440 monetary units (abbreviated m.u.). He gives the salesman a 10001000 m.u. bill, and receives, in change, 340340 m.u. The base rr is:
22
55
77
88
1212
Answer: D
Concepts:number baseplace valuequadratic
Small hint:
Translate 1000r−440r=340r1000_r-440_r=340_r into powers of rr
Big hint:
Simplify the resulting quadratic and use a base larger than every digit shown
Solution:
In ordinary notation the transaction says r3−(4r2+4r)=3r2+4r. r^3-(4r^2+4r)=3r^2+4r. Thus r(r2−7r−8)=0,r(r^2-7r-8)=0, so r=8r=8 or r=−1.r=-1. A numeral base must be positive and exceed 4,4, hence r=8.r=8.
Therefore, the correct answer is D.
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