Let $N$ be the smallest positive integer such that among any $N$ consecutive integers, at least one is coprime to $374 = 2 \times 11 \times 17$. Find $N$.
(A) 4 (B) 5 (C) 6 (D) 7

In the Mathematics department of a college, there are 60 first year students, 84 second year students, and 108 third year students. All of these students are to be divided into project groups such that each group has the same number of first year students, the same number of second year students, and the same number of third year students. What is the smallest possible size of each group?
(A) 9
(B) 12
(C) 19
(D) 21.

Consider a right-angled triangle with integer-valued sides $a < b < c$ where $a, b, c$ are pairwise co-prime. Let $d = c - b$. Suppose $d$ divides $a$. Then
(a) Prove that $d \leq 2$.
(b) Find all such triangles (i.e. all possible triplets $a, b, c$) with perimeter less than 100.

The number of ways one can express $2 ^ { 2 } 3 ^ { 3 } 5 ^ { 5 } 7 ^ { 7 }$ as a product of two numbers $a$ and $b$, where $\operatorname { gcd } ( a , b ) = 1$, and $1 < a < b$, is
(A) 5 .
(B) 6 .
(C) 7 .
(D) 8 .

Define $a = p ^ { 3 } + p ^ { 2 } + p + 11$ and $b = p ^ { 2 } + 1$, where $p$ is any prime number. Let $d = \operatorname { gcd } ( a , b )$. Then the set of possible values of $d$ is
(A) $\{ 1,2,5 \}$.
(B) $\{ 2,5,10 \}$.
(C) $\{ 1,5,10 \}$.
(D) $\{ 1,2,10 \}$.

Let $n > 1$ be the smallest composite integer that is coprime to $\frac{10000!}{9900!}$. Then
(A) $n \leqslant 100$
(B) $100 < n \leqslant 9900$
(C) $9900 < n \leqslant 10000$
(D) $n > 10000$

8. If $r , s , t$ are prime numbers and $p , q$ are the positive integers such that the LCM of $p , q$ is $r ^ { 2 } t ^ { 4 } s ^ { 2 }$, then the number of ordered pair ( $p , q$ ) is
(A) 252
(B) 254
(C) 225
(D) 224
Sol. (C) Required number of ordered pair (p,q) is $( 2 \times 3 - 1 ) ( 2 \times 5 - 1 ) ( 2 \times 3 - 1 ) = 225$.

The sum of all natural numbers $n$ such that $100 < n < 200$ and H.C.F.$(91, n) > 1$ is
(1) 3203
(2) 3221
(3) 3121
(4) 3303

Let $A = \{ 2,3,4,5 , \ldots , 30 \}$ and $\sim$ be an equivalence relation on $A \times A$, defined by $( a , b ) \simeq ( c , d )$, if and only if $ad = bc$. Then the number of ordered pairs which satisfy this equivalence relation with ordered pair $( 4,3 )$ is equal to :
(1) 5
(2) 6
(3) 8
(4) 7

Let $A = \{ n \in N :$ H.C.F.$( n , 45 ) = 1 \}$ and let $B = \{ 2k : k \in \{ 1 , 2 , \ldots , 100 \} \}$. Then the sum of all the elements of $A \cap B$ is $\_\_\_\_$.

If $\operatorname{gcd}(m, n) = 1$ and $1^{2} - 2^{2} + 3^{2} - 4^{2} + \ldots + (2021)^{2} - (2022)^{2} + (2023)^{2} = 1012m^{2}n$ then $m^{2} - n^{2}$ is equal to
(1) 240
(2) 200
(3) 220
(4) 180

Let $A = \{ 1,2,3 , \ldots , 10 \}$ and $B = \left\{ \frac { m } { n } : m , n \in A , m < n \right.$ and $\left. \operatorname { gcd } ( m , n ) = 1 \right\}$. Then $n ( B )$ is equal to:
(1) 36
(2) 31
(3) 37
(4) 29

Let $a$ and $b$ be natural numbers such that the greatest common divisor of $a$ and $b$ is 3. We are to find the natural numbers $a$ and $b$ such that
$$3 a - 2 b = \ell + 3$$
is satisfied, where $\ell$ is the least common multiple of $a$ and $b$.
When we set $a = 3 p$ and $b = 3 q$, the natural numbers $p$ and $q$ are mutually prime (co-prime), and hence $\ell = \mathbf { N } p q$. Thus using $p$ and $q$, the equality (1) can be transformed to
$$p q - \mathbf { O } p + \mathbf { P } q + \mathbf { Q } = 0 .$$
This can be further transformed to
$$( p + \mathbf { R } ) ( q - \mathbf { S } ) = - \mathbf { S } \mathbf { T } .$$
Among the pairs of integers $p$ and $q$ which satisfy this equation, the pair such that both $a$ and $b$ are natural numbers is
$$p = \mathbf { U } , \quad q = \mathbf { V } ,$$
which gives
$$a = \mathbf { W X } , \quad b = \mathbf { Y } .$$

6. Suppose $a , b , c$ are three positive integers. If 25 is the greatest common divisor of $a$ and $b$, and $3, 4, 14$ are all common divisors of $b$ and $c$, which of the following is correct?
(1) $c$ must be divisible by 56.
(2) $b \geq 2100$.
(3) If $a \leq 100$, then $a = 25$.
(4) The greatest common divisor of $a , b , c$ is a divisor of 25.
(5) The least common multiple of $a , b , c$ is greater than or equal to $25 \times 3 \times 4 \times 14$.

On the set of positive integers, the operations $\oplus$ and $\otimes$ are defined using the greatest common divisor and least common multiple as follows:
$$\begin{aligned} & a \oplus b = \operatorname { GCD } ( a , b ) \\ & a \otimes b = \operatorname { LCM } ( a , b ) \end{aligned}$$
Accordingly, what is the result of the operation $18 \oplus ( 12 \otimes 4 )$?
A) 2
B) 3
C) 6
D) 8
E) 9

The least common multiple of $b$ and $40$ is $120$.
Accordingly, how many different positive integers $b$ are there?
A) 6
B) 8
C) 10
D) 12
E) 14

Let p and q be distinct prime numbers such that
$$\begin{aligned} & a = p ^ { 4 } \cdot q ^ { 2 } \\ & b = p ^ { 2 } \cdot q ^ { 3 } \end{aligned}$$
Which of the following is the greatest common divisor of numbers a and b?
A) $p ^ { 5 } \cdot q ^ { 4 }$
B) $p ^ { 4 } \cdot q ^ { 3 }$
C) $p ^ { 3 } \cdot q ^ { 4 }$
D) $p ^ { 2 } \cdot q ^ { 2 }$
E) $p ^ { 2 } \cdot q ^ { 3 }$

Let n be a positive integer, and let $S(n)$ denote the set of positive integers that divide n without remainder.
Accordingly, how many elements does the intersection set $S(60) \cap S(72)$ have?
A) 8 B) 9 C) 6 D) 5 E) 4

$\mathbf { a } < \mathbf { b } < \mathbf { c }$ are positive integers and
$$\begin{aligned} & \gcd ( a , b ) = 5 \\ & \gcd ( b , c ) = 4 \end{aligned}$$
Given this, what is the minimum value that the sum $\mathbf { a + b + c }$ can take?
A) 27
B) 35
C) 39
D) 45
E) 49

Let $a$ and $b$ be distinct positive integers such that LCM(a,b) equals a prime number.
Accordingly,\ I. $a$ and $b$ are coprime numbers.\ II. The sum $a + b$ is an odd number.\ III. The product $\mathrm{a} \cdot \mathrm{b}$ is an odd number.
Which of the following statements are always true?\ A) Only I\ B) Only II\ C) Only III\ D) I and II\ E) II and III

Let p, r, and t be different prime numbers;

• Integer multiples of p form set A,
• Integer multiples of r form set B,
• Integer multiples of t form set C.

It is known that two of the numbers 220, 245, 330, and 350 are elements of the blue-colored set, and the other two are elements of the yellow-colored set. Accordingly, what is the sum $\mathbf { p } + \mathbf { r } + \mathbf { t }$?
A) 10
B) 14
C) 15
D) 21
E) 23

Let $m$ and $n$ be positive integers such that
$$\begin{aligned} & \gcd ( m , n ) + \text{lcm} ( m , n ) = 289 \\ & m + n \neq 289 \end{aligned}$$
What is the sum $m + n$?
A) 41
B) 43
C) 45
D) 47
E) 49

Let $a, b, c$ and $d$ be positive integers. $$\begin{aligned} & M = 6^{a} \cdot 5^{b} \\ & N = 10^{c} \cdot 9^{d} \end{aligned}$$ For the numbers $M$ and $N$ $$\begin{aligned} & \gcd(M, N) = 2^{3} \cdot 3^{2} \cdot 5 \\ & \text{lcm}(M, N) = 2^{5} \cdot 3^{3} \cdot 5^{5} \end{aligned}$$ are given. Accordingly, what is the sum $a + b + c + d$?
A) 8 B) 9 C) 10 D) 11 E) 12