Singapore Mathematics Olympiad - 2020- Junior Years - Questions

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Multiple Choice Questions
Problem 01:

Let \(x=2^{300}, y=3^{200}\) and \(z=6^{100}\). Which of the following is true?
(A) \(x>y>z\)
(B) \(x>z>y\)
(C) \(y>z>x\)
(D) \(y>x>z\)
(E) \(z>x>y\)

Problem 02:

An expensive painting was stolen and the police rounded up five suspects Alfred, Boris, Chucky, Dan and Eddie. These were the statements that were recorded.
Alfred: "Either Boris or Dan stole the painting."
Boris: "I think Dan or Eddie is the guilty party."
Chucky: "It must be Dan."
Dan: "Boris or Eddie did it!"
Eddie: "I am absolutely sure the thief is Alfred."
The police knew that only one of the five suspects stole the painting and that all five were lying. Who stole the painting?
(A) Alfred
(B) Boris
(C) Chucky
(D) Dan
(E) Eddie

Problem 03:

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 \(\square<\triangle\) to represent \(\square\) is lighter than \(\triangle\), which of the following is true?

(A) \(\square<0<\triangle\)
(B) \(\square<\triangle<0\)
(C) \(\triangle<\square<0\)
(D) \(\triangle<0<\square\)

Problem 04:

The integer 6 has exactly four positive factors, namely \(1,2,3\) and \(6\) . Likewise, the integer \(8\) has exactly four positive factors, \(1,2,4\) and \(8\) . How many integers from \(9\) to \(50\) (inclusive) have exactly four positive factors?
(A) \(10\)
(B) \(11\)
(C) \(12\)
(D) \(13\)
(E) \(14\)

Problem 05:

A company sells a bottle of juice at \(\$ 1\) per bottle and offers to give you a free bottle of juice if you recycle four empty bottles. Assuming there is no limit to the offer and you can recycle bottles that you receive from the offer, what is the largest number of bottles you can eventually consume if you have \(\$ 43\) ?
(A) 53
(B) 54
(C) 55
(D) 56
(E) 57

Short Questions
Problem 06:

Let \(n\) be a positive integer. Suppose the lowest common multiple of \(4,5\) and \(n\) is \(2020\) . What is the sum of the smallest possible value of \(n\) and the largest possible value of \(n\) ?

Problem 07:

When the five-digit integer \(\overline{2 x 6 y x}\) is divided by the four-digit integer \(\overline{5 y 27}\), the quotient is 4 and remainder is \(\overline{x 106}\), which is a four-digit integer. What is the value of the digit \(x\) ?

Problem 08:

How many multiples of 11 are there in the sequence \(1100,1101,1102, \ldots, 2019,2020\) ?

Problem 09:

A quadruple \((a, b, c, d)\) of positive integers is skewed if the median and mode of \(a, b, c, d\) are equal, but strictly greater than the mean of \(a, b, c, d\). How many skewed \((a, b, c, d)\) of positive integers are there that satisfy \(a \leq b \leq c \leq d\) and \(a+b+c+d=40\) ?

Problem 10:

Let \(T_n\) be a linear sequence given by
\[
T_1=10, T_2=13, T_3=16, \ldots,
\]
and \(S_n\) be a quadratic sequence given by
\[
S_1=10, S_2=23, S_3=39, \ldots
\]

What is the value of \(S_{101}-T_{101}\) ?

Problem 11:

Let \(A B C\) be a triangle where \(D\) is the midpoint of \(B C\) and \(E\) lies on \(A C\) such that \(A E: E C=3: 1\). Let \(F\) be the intersection of \(A D\) and \(B E\). If the area of \(A B C\) is \(280\) , what is the area of triangle \(B F D\) ?

Problem 12:

In the figure below, each distinct letter represents a unique distinct digit such that the arithmetic holds. If the letter K represents 6 , what number does SHAKE represent?

Problem 13:

The diagram below shows a shape formed from four semi-circles whose radii are distinct integers. If the shape has perimeter of \(18 \pi\) and encloses an area of \(k \pi\), where \(k\) is a prime, what is the value of \(k\) ?

Problem 14:

If the sum of the two diagonals of a kite is \(100\) , what is the largest possible area of the kite?

Problem 15:

What is the smallest five-digit integer of the form \(\overline{5 x 20 y}\) that is divisible by \(33\) ?

Problem 16:

What is the value of \(2020^3-9(2020)(2017)-2017^3\) ?

Problem 17:

In the following diagram, \(A B C D\) is a rectangle where \(E\) and \(F\) are points on \(B C\) and \(C D\) respectively. The area of triangle \(A E F\), denoted \([A E F]\), is \(2037\) . If
\[
[A E C F]=2[A B E]=3[A D F],
\]
what is the area of the rectangle \(A B C D\) ?

Problem 18:

In the following diagram, \(A B C D\) is a square of side length \(64\) . \(E\) is the midpoint of \(A B\), \(F\) is the midpoint of \(E C\) and \(G\) is the midpoint of \(F D\). What is the area enclosed by the quadrilateral \(A E F G\) ?

Problem 19:

Let \(X=1234 \cdots 78798081\) be the integer that consists of all the integers from \(1\) to \(81\) written from left to right. What is the remainder of \(X\) when divided by \(2020\) ?

Problem 20:

Let \(A=\frac{1}{7} \times 3.14 \dot{1} \dot{5}\), where \(3.14 \dot{1} \dot{5}\) is the rational number with recurring digits \(15\) . In other words,
\[
3.14 \dot{1} \dot{5}=3.14+0.0015+0.000015+0.00000015+\cdots \] Suppose that \(A=\frac{m}{n}\), where \(m\) and \(n\) are positive integers with no common factors larger than \(1\) . What is the value of \(m+n\) ?

Problem 21:

Ali and Barry went running on a standard \(400\) metre track. They started simultaneously at the same location on the track but ran in opposite directions. Coincidentally, after \(24\) minutes, they ended at the same location where they started. Ali completed \(12\) rounds of the track in those \(24\) minutes while Barry completed \(10\) rounds. How many times did Ali and Barry pass each other during the run? (Exclude from your answer the times that they met at the start of the of run and when they completed the run after \(24\) minutes.)

Problem 22:

If we have
\[
(1-3 x)+(1-3 x)^2+\cdots+(1-3 x)^{100}=a_0+a_1 x+a_2 x^2+\cdots+a_{100} x^{100},
\]
for some integers \(a_0, a_1, \ldots, a_{100}\), what is the value of
\[
\left|\frac{a_1}{3}+\frac{a_2}{3^2}+\cdots+\frac{a_{100}}{3^{100}}\right| ?
\]

Problem 23:

What is the value of \(\sqrt{27-\sqrt{704}}+\sqrt{27+\sqrt{704}}\) ?

Problem 24:

If \(m>n\) are positive integers satisfying
\[
\left(m^2-n^2\right)^2=1+80 n,
\]
what is the smallest possible value of \(mn\) ?

Problem 25:

What is the value of
\[\left(\frac{1}{2}+\frac{1}{3}\right+\frac{1}{4}+\frac{1}{5}+\ldots+\frac{1}{37}\right)+\left(\frac{2}{3}+\frac{2}{4}+\frac{2}{5}+\ldots+\frac{2}{37}\right)+\left(\frac{3}{4}+\frac{3}{5}+\frac{3}{6}+\ldots+\frac{3}{37}\right)+\ldots+\left(\frac{35}{36}+\frac{35}{37}\right)+\frac{36}{37}=?\]

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