Transcription of Modular Arithmetic Practice - CMU
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Modular Arithmetic PracticeJoseph ZollerSeptember 13, 2015 Practice Problem Solutions1. Given that 5x 6 (mod 8), findx.[Solution: 6]2. Find the last digit of 7100[Solution: 1]7100 (72)50 4950 ( 1)50 1 mod (1992 AHSME 17) The two-digit integers form 19 to 92 are written consecutively to form thelarge integerN= 192021 that 3kis the highest power of 3 that is a factor ofN. What isk?[Solution:k= 1]We know thatN S(N) mod (N) = 1+9+9+0+9+1+9+2+10(2+..+8)+7(0+..9) =40 + 10(35) + 7(45) = 40 + 350 + 315 = 705. ThenN S(N) S(S(N)) S(705) 12 3mod 9. Thus, it is only divisible by 3 and not 9, andk= (2000 AMC 12 18) In yearN, the 300th day of the year is a Tuesday. In yearN+ 1, the 200thday is also a Tuesday. On what day of the week did the 100th day of the yearN 1 occur?[Solution: Thursday]There are either 65 + 200 = 265 or 66 + 200 = 266 days between the first two dates dependingupon whether or not yearNis a leap year.
Sep 13, 2015 · 12. When 30! is computed, it ends in 7 zeros. Find the digit that immediately precedes these zeros. [Solution: 8] 30!=107 = 226 2314 157 174 711 4132 217 191 23 291=10 = 219 314 7 112 13 17 1 119 231 29 . So, 30! = 219 143 174 1112 1132 17 19 23 291 8 9 1 1 9 7 9 3 9 ( 2) ( 1) ( 1) ( 3) ( 1) 3 ( 1) 18 8 mod 10.
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