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Algebra Difficulty 2.9 Junior Find the answer

What is the sum of all numbers qq which can be written in the form q=abq=\frac{a}{b} where aa and bb are positive integers with b10b \leq 10 and for which there are exactly 19 integers nn that satisfy q<n<q\sqrt{q}<n<q?

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Suppose that a number qq has the property that there are exactly 19 integers nn with q<n<q\sqrt{q}<n<q. Suppose that these 19 integers are m,m+1,m+2,,m+17,m+18m, m+1, m+2, \ldots, m+17, m+18. Then q<m<m+1<m+2<<m+17<m+18<q\sqrt{q}<m<m+1<m+2<\cdots<m+17<m+18<q. This tells us that qq>(m+18)m=18q-\sqrt{q}>(m+18)-m=18 because qqq-\sqrt{q} is as small as possible when qq is as small as possible and q\sqrt{q} is as large as possible. Also, since this is exactly the list of integers that is included strictly between q\sqrt{q} and qq, then we must have m1q<m<m+1<m+2<<m+17<m+18<qm+19m-1 \leq \sqrt{q}<m<m+1<m+2<\cdots<m+17<m+18<q \leq m+19. In other words, neither m1m-1 nor m+19m+19 can satisfy q<n<q\sqrt{q}<n<q. This tell us that qq(m+19)(m1)=20q-\sqrt{q} \leq(m+19)-(m-1)=20. Therefore, we have that 18<qq2018<q-\sqrt{q} \leq 20. Next, we use 18<qq2018<q-\sqrt{q} \leq 20 to get a restriction on qq itself. To have qq>18q-\sqrt{q}>18, we certainly need q>18q>18. But if q>18q>18, then q>18>4\sqrt{q}>\sqrt{18}>4. Furthermore, qq>18q-\sqrt{q}>18 and q>4\sqrt{q}>4 give q4>qq>18q-4>q-\sqrt{q}>18 and so q>22q>22. Next, note that qq=q(q1)q-\sqrt{q}=\sqrt{q}(\sqrt{q}-1). When qq is larger than 1 and increases, each factor q\sqrt{q} and q1\sqrt{q}-1 increases, so the product qqq-\sqrt{q} increases. When q=25,qq=255=20q=25, q-\sqrt{q}=25-5=20. Since we need qq20q-\sqrt{q} \leq 20 and since qq=20q-\sqrt{q}=20 when q=25q=25 and since qqq-\sqrt{q} is increasing, then for qq20q-\sqrt{q} \leq 20, we must have q25q \leq 25. Therefore, 18<qq2018<q-\sqrt{q} \leq 20 tells us that 22<q2522<q \leq 25. So we limit our search for qq to this range. When q=22,q4.69q=22, \sqrt{q} \approx 4.69, and so the integers nn that satisfy q<n<q\sqrt{q}<n<q are 5,6,7,,20,215,6,7, \ldots, 20,21, of which there are 17. When 22<q2322<q \leq 23, we have 4<q<54<\sqrt{q}<5 and 22<q2322<q \leq 23, which means that the integers nn that satisfy q<n<q\sqrt{q}<n<q are 5,6,7,,20,21,225,6,7, \ldots, 20,21,22, of which there are 18. When 23<q2423<q \leq 24, we have 4<q<54<\sqrt{q}<5 and 23<q2423<q \leq 24, which means that the integers nn that satisfy q<n<q\sqrt{q}<n<q are 5,6,7,,20,21,22,235,6,7, \ldots, 20,21,22,23, of which there are 19. When 24<q<2524<q<25, we have 4<q<54<\sqrt{q}<5 and 24<q<2524<q<25, which means that the integers nn that satisfy q<n<q\sqrt{q}<n<q are 5,6,7,,20,21,22,23,245,6,7, \ldots, 20,21,22,23,24, of which there are 20. When q=25,q=5q=25, \sqrt{q}=5 and so the integers that satisfy q<n<q\sqrt{q}<n<q are 6,7,,20,21,22,23,246,7, \ldots, 20,21,22,23,24, of which there are 19. Therefore, the numbers qq for which there are exactly 19 integers nn that satisfy q<n<q\sqrt{q}<n<q are q=25q=25 and those qq that satisfy 23<q2423<q \leq 24. Finally, we must determine the sum of all such qq that are of the form q=abq=\frac{a}{b} where aa and bb are positive integers with b10b \leq 10. The integers q=24q=24 and q=25q=25 are of this form with a=24a=24 and a=25a=25, respectively, and b=1b=1. The qq between 23 and 24 that are of this form with b4b \leq 4 are 2312=472,2313=703,2323=713,2314=934,2334=95423 \frac{1}{2}=\frac{47}{2}, 23 \frac{1}{3}=\frac{70}{3}, 23 \frac{2}{3}=\frac{71}{3}, 23 \frac{1}{4}=\frac{93}{4}, 23 \frac{3}{4}=\frac{95}{4}. Notice that we don't include 232423 \frac{2}{4} since this is the same as the number 231223 \frac{1}{2}. We continue by including those satisfying 5b105 \leq b \leq 10 and not including equivalent numbers that have already been included with smaller denominators, we obtain 2312,2313,2323,2314,2334,2315,2325,2335,2345,2316,2356,2317,2327,2337,2347,2357,2367,2318,2338,2358,2378,2319,2329,2349,2359,2379,2389,23110,23310,23710,2391023 \frac{1}{2}, 23 \frac{1}{3}, 23 \frac{2}{3}, 23 \frac{1}{4}, 23 \frac{3}{4}, 23 \frac{1}{5}, 23 \frac{2}{5}, 23 \frac{3}{5}, 23 \frac{4}{5}, 23 \frac{1}{6}, 23 \frac{5}{6}, 23 \frac{1}{7}, 23 \frac{2}{7}, 23 \frac{3}{7}, 23 \frac{4}{7}, 23 \frac{5}{7}, 23 \frac{6}{7}, 23 \frac{1}{8}, 23 \frac{3}{8}, 23 \frac{5}{8}, 23 \frac{7}{8}, 23 \frac{1}{9}, 23 \frac{2}{9}, 23 \frac{4}{9}, 23 \frac{5}{9}, 23 \frac{7}{9}, 23 \frac{8}{9}, 23 \frac{1}{10}, 23 \frac{3}{10}, 23 \frac{7}{10}, 23 \frac{9}{10}. There are 31 numbers in this list. Each of these 31 numbers equals 23 plus a fraction between 0 and 1. With the exception of the one number with denominator 2, each of the fractions can be paired with another fraction with the same denominator to obtain a sum of 1. Therefore, the sum of all of these qq between 23 and 24 is 31(23)+12+15(1)=7281231(23)+\frac{1}{2}+15(1)=728 \frac{1}{2}, because there are 31 contributions of 23 plus the fraction 12\frac{1}{2} plus 15 pairs of fractions with a sum of 1. Finally, the sum of all qq of the proper form for which there are exactly 19 integers that satisfy q<n<q\sqrt{q}<n<q is 72812+25+24=77712728 \frac{1}{2}+25+24=777 \frac{1}{2}.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.