Selection with Arithmetic or Divisibility Conditions

The question requires selecting numbers from a set such that their sum, product, or other arithmetic property satisfies a given condition (e.g., sum is a multiple of 3, product is a perfect square).

csat-suneung 2006 Q28 4 marks View
When selecting two different odd numbers from the odd numbers from 1 to 30, how many cases are there where the sum of the two numbers is a multiple of 3? [4 points]
(1) 43
(2) 41
(3) 39
(4) 37
(5) 35
csat-suneung 2016 Q14 4 marks View
For three integers $a , b , c$ satisfying $$1 \leq | a | \leq | b | \leq | c | \leq 5$$ what is the number of all ordered pairs $( a , b , c )$? [4 points]
(1) 360
(2) 320
(3) 280
(4) 240
(5) 200
gaokao 2022 Q6 5 marks View
From 6 cards labeled $1,2,3,4,5,6$ respectively, 2 cards are randomly drawn without replacement. The probability that one of the drawn numbers is a multiple of the other is
A. $\frac { 1 } { 5 }$
B. $\frac { 1 } { 3 }$
C. $\frac { 2 } { 5 }$
D. $\frac { 2 } { 3 }$
jee-main 2021 Q82 View
Let $S = \{ 1,2,3,4,5,6,9 \}$. Then the number of elements in the set $T = \{ A \subseteq S : A \neq \phi$ and the sum of all the elements of $A$ is not a multiple of $3 \}$ is
taiwan-gsat 2021 QC 5 marks View
From the nine numbers 1, 2, 3, 4, 5, 6, 7, 8, 9, any three distinct numbers are randomly selected, with each number having equal probability of being selected. The probability that the product of the three numbers is a perfect square is (20). (Express as a fraction in lowest terms)
turkey-yks 2023 Q29 View
In a course, the weekly lesson durations of 7 lessons, each with different lesson times, are given in the table below.
LessonDuration (hours)
Lesson 15
Lesson 24
Lesson 34
Lesson 45
Lesson 53
Lesson 65
Lesson 75

Aslı, who enrolled in this course, wants to take four different lessons such that the total weekly lesson duration is 17 hours.
Accordingly, in how many different ways can Aslı select the lessons she will take?
A) 8 B) 10 C) 12 D) 16 E) 18