2003 Solutions Euclid Contest - CEMC - University of Waterloo

2003 Euclid Solutions 6This total still included some multiples of 10 that are less or equal to 300 (but not all,since we have already removed 30, for instance).In fact, there are 30 multiples of 10 less than or equal 300, 10 of which are multiples of15 as well (that is, the multiples of 30). So we must remove 20 from the total of 120.We then obtain that there are 100 positive integers less than or equal to 300 which aremultiples of 3 or 5, but not of 10 or 15.5. (a) Since the signs alternate every three terms, it makes sense to look at the terms in groupsof 6.The sum of the first 6 terms is 1 + 3 + 5 −7 −9 − 11 = − 18.The sum of the next 6 terms is 13 + 15 + 17 −19 −21 − 23 = − 18.In fact, the sum of each group of 6 terms will be the same, since in each group, 12 hasbeen added to the numerical value of each term when compared to the previous group of6, so overall 12 has been added three times and subtracted three times.Since we are looking for the sum of the first 300 terms, then we are looking at 50 groupsof 6 terms, so the sum must be 50( −18)=− 900.Answer: −900(b) Let the two digit integer have tens digit a and units digit b. Then the given informationtells us2 2a + 10b = b + 10a2 2a −b − 10a+ 10b= 0( a+b) ( a−b)−10( a−b)=0( a−b) ( a+ b−10)=0and so a = b or a+ b =10.So the possibilities for the integer are 11, 22, 33, 44, 55, 66, 77, 88, 99, 19, 28, 37, 46, 55,64, 73, 82, 91. We now must determine which integers in this list are prime.We can quickly reject all multiples of 11 bigger than 11 and all of the even integers, toreduce the list to 11, 19, 37, 73, 91.All of these are prime, except for 91 = 13 × 7.Therefore, the required integers are 11, 19, 37, and 73.6. (a) Solution 1In 24 minutes, the number of atoms of isotope A has halved 4 times, so the initial numberof atoms is 2 4 = 16 times the number of atoms of isotope A at time 24 minutes.But there were initially half as many atoms of isotope B as of isotope B, so there was 8times the final number of atoms. Therefore, the number of atoms of isotope B halves 3times in the 24 minutes, so it takes 8 minutes for the number of atoms of isotope B tohalve.