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Singapore Mathematical Society Singapore Mathematical Olympiad (SMO) 2021 Junior Section (Round 1) Wednesday, 2 June 2021 0930-1200 hrs Instructions to contestants 1. Answer ALL 25 questions. 2. Enter your answers on the answer sheet provided. 3. For the multiple choice questions, enter your answer on the answer sheet by shading the bubble containing the letter (A, B, C, Dor E} corresponding to the correct answer. 4. For the other short questions, write your answer on the answer sheet and shade the ap- propriate bubble below your answer. 5. No steps are needed to justify your answers . 6. Each question carries 1 mark. 7. No calculators are allowed. 8. Throughout this paper, let L x」 denote the greatest integer less than or 叨ual to x. For example, L2.l 」= 2, L3.9 」=3. PLEASE DO NOT TURN OVER UNTIL YOU ARE TOLD TO DO SO. 1
Multiple Choice Questions l. Let a and b be real numbers satisfying a < 0 < b. Which of the following is not true? (A) a2b > O (B) ab2 < O (C) 7; > 0 (D) b- a> O (E) la - bl> 0 2. The following diagram shows a system of balances hanging from the ceiling with three types of weights. The balances tip down to the heavier side. If .we use 口< !::, to represent 口 is lighter than !::,, which of the following is true? A^] X (D) y > X > z (E) z > X > y 4. In the diagram, six circles are tangent to each other. If the radius of the largest circle is 1 and the radii of the four medium sized circles are equal, what is the radius of the smallest circle? (A) 迈- 1 (B) 3 - 2迈 (C) 2 -迈 (D) 6- 4迈 (E) None of the above 5. Which of the following is closest to the value of 1 + 1 + 1 1 ? 迈 +1 没＋迈 v'4+ 没 十．．．十一一· (A) 10 (B) 20 (C) 30 (D) 40 (E) 50 2
Short Questions 6. Let x be a positive integer. Suppose that the lowest common multiple of x and 14 is 42 and the lowest common multiple of x and 33 is 66 . What is the value of x? 7 . What are the last four digits of the sum 1 + 22 + 333 + 4444 +· · · 十 999999999?-"~, nine 9s 't'l 05 Give your answer as a 4-digit number. 8 . How many distinct triples of positive integers (a, b, c) satisfy 1 ,,;; a ,,;; b,,;; c and 1 1 1 -+-+-=1? a b c 9. Given five consecutive positive integers, if the product of the largest and the smallest integer is 2021, what is the sum of the five integers? 10 . The numbers from 1 to 2021 are concatenated from left to right and the result is read as an integer 12345678910111213· · ·201920202021. What is the remainder when this number is divided by 6? 11. In 'the figure below, each distinct letter represents a unique distinct digit such that the arithmetic holds. If S represents 6 and E represents 8, ,what number does SIX represent? 尽 沧 沁 ~ lN 、 袍 1\/ 8E 1N +~ 勹 sx IT 货 ~ 2N•T w 12 . What is the value of ✓ (219)(220)(221)(222) + 1? 13 . Let A, B, ... , I be unknowns satisfying A.+B+C= 1, B+C+D = 2, C+D+E=3, D+E+F=4, E+F+G=5, F+G+H=6, G+H+I=7. What is the value of A+ E + I? 3
14. If xis a 3-digit number, we define M(x) and m(x) respectively as the largest and smallest positive number that can be formed by rearranging the three digits of x. For example, if x = 123 , then M(123) = 321 and m(123) = 123. If y = 898, then M(898) = 988 and m(898) = 889. Given that z is a 3-digit number that satisfies z = M(z) -m(z) , what is the value of z? 15 . How many integers k are there such that the quadratic equation kx2 + 20x + 20 -k = 0 has only integer solutions? 16. In the following diagram, ABCD is a quadrilateral inscribed in a circle with centre 0. If JABI = JBCI = 6, JADI = 14 and CD is a diameter, what is the length of JCDJ? B 17 . The diagram below shows a piece of cardboard in the shape of an equilateral triangle with side leRgth 36 cm. Six perpendicular cuts of length 2v'3 cm are made to remove the corners in orde r to fold the cardboard into a tray whose base is an equilateral triangle and height is 2沟 cm. What is the volume of the tray in cm 砂 矣，I ，、I I ，、I ,、 ,' I ,'、｀, 2戎 cm ,`` .. _______ ------ ... ◄ ------------ --------+ 36 cm 4
~ 18. What is the value of l五言－二 j ? 19. Let x be the positive real number that satisfies V吐 -4x+5 + v'丘2+ 4x + 5 = 3x. What is the value of l 10 灶勹？ 20. What is the number of· · pos1t1ve mtegers c such that the equation x2 - 2021x + 100c = 0 has real roots? 21. In chess, two queens ar~said to be attacking each other if they are positioned in the same row , column or diagonal on a chessboard. How many ways are there to place two identical queens in a 4 x 4 chessboard such that they do not attack each other? 沪~'\, 心如 X . 戏 o\:: 咕 S ｀罪 嗡 3 5 7 801 A 22. Let A = 2 x 4 x 6 x· · ·x 颈 What is the value of l司？ 23. A 3 x 3 grid is filled with the integers 1 to 9. An arrangement is nicely ordered if the integers in each horizontal row is incre 邸ing from left to right and the integers in each vertical column is incre 函 ng from top to bottom. Two examples ot nicely ordered arrangements are given in the diagram below. What is the total number of distinct nicely ordered arrangements? 1 3 7 2 4 8 5 6 9 1 2 6 3 5 8 4 7 9 24. A class has exactly 50 students and it is known that 40 students scored A in English, 45 scored A in Mathematics and 42 scored A in Science . What is the minimum number students who scored A in all three subjects? uC (八 "'-S 25. Suppose a positive integer x satisfies the following equation ~x + 76638 - ifx -76637 = 5. What is the value of x? 5