1995 AMC 8 Problem 12

Attempt Problem 12 of the 1995 AMC 8 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 1995 AMC 8 solutions, or check the answer key.

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

12.

A lucky year is one in which at least one date, when written in the form month/day/year, has the following property: the product of the month times the day equals the last two digits of the year. For example, 19561956 is a lucky year because it has the date 7/8/567/8/56 and 7×8=56.7 \times 8 = 56. Which of the following is NOT a lucky year?
19901990
19911991
19921992
19931993
19941994
Answer: E
Concepts:factorcasework
Small hint:
For each year, try to write its last two digits as (month) ×\times (day) with a valid month and day
Big hint:
A year fails only when the last two digits cannot be a product of a number at most 1212 and a number at most 3131
Solution:
Each of the other years works: 90=9×10,90 = 9 \times 10, 91=7×13,91 = 7 \times 13, 92=4×23,92 = 4 \times 23, and 93=3×31,93 = 3 \times 31, each a valid month/day.
For 1994,1994, the last two digits factor only as 94=2×47,94 = 2 \times 47, and 4747 is too large for a day (and 9494 or 4747 is too large for a month). So 19941994 has no lucky date.
Thus, the correct answer is E .
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