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Which of the following numbers are prime and which are composite?

101,102,105,109,112,113,120,121,123,127,131.

Important Questions on Playing with Numbers

EASY
Mathematics>Arithmetic>Playing with Numbers>Prime and Composite Numbers
Find the smallest positive integer n≥10 such that n+6 is a prime and 9n+7 is a perfect square.
EASY
Mathematics>Arithmetic>Playing with Numbers>Prime and Composite Numbers
A two-digit number ab¯ is called almost prime if one obtains a two-digit prime number by changing at most one of its digits a & b. (For example, 18 is an almost prime number because 13 is a prime number). Then the number of almost prime two-digit numbers is
EASY
Mathematics>Arithmetic>Playing with Numbers>Prime and Composite Numbers
There are four prime numbers written in ascending order. The product of the first three is 385 and that of the last three is 1001. The last number is :
HARD
Mathematics>Arithmetic>Playing with Numbers>Prime and Composite Numbers
The prime numbers a, b and c are such that a+b2=4c2. Determine the sum of all possible values of a+b+c.
MEDIUM
Mathematics>Arithmetic>Playing with Numbers>Prime and Composite Numbers
For an integer n, let Sn=n+1,n+2,…., n+18. Which of the following is true for all n≥10 ?
HARD
Mathematics>Arithmetic>Playing with Numbers>Prime and Composite Numbers
Let N denote the number of all natural numbers n such that n is divisible by a prime p>n and p<20. What is the value of N?
EASY
Mathematics>Arithmetic>Playing with Numbers>Prime and Composite Numbers
A prime number p is called special if there exist primes p1, p2, p3, p4 such that p=p1+p2=p3-p4.
The number of special primes is
 
EASY
Mathematics>Arithmetic>Playing with Numbers>Prime and Composite Numbers
Express 120 as a product of its prime factors.
EASY
Mathematics>Arithmetic>Playing with Numbers>Prime and Composite Numbers
Which of the following number is an odd and prime number?
EASY
Mathematics>Arithmetic>Playing with Numbers>Prime and Composite Numbers
The sum of the squares of the digits of the largest two-digit prime number is:
EASY
Mathematics>Arithmetic>Playing with Numbers>Prime and Composite Numbers
If a and b are positive integers such that a2-b2 is a prime number, then a2-b2 is equal to:
HARD
Mathematics>Arithmetic>Playing with Numbers>Prime and Composite Numbers
The number of 6-digit numbers of the form ababab (in base 10) each of which is a product of exactly 6 distinct primes is
EASY
Mathematics>Arithmetic>Playing with Numbers>Prime and Composite Numbers
It is given that the number 43361 can be written as a product of two distinct prime numbers p1, p2. Further, assume that there are 42900 numbers which are less than 43361 and are co-prime to it. Then, p1+p2 is
MEDIUM
Mathematics>Arithmetic>Playing with Numbers>Prime and Composite Numbers
Consider the set E={5,6,7,8,9}. For any partition {A,B} of E, with both A and B non-empty, consider the number obtained by adding the product of elements of A to the product of elements of B. Let N be the largest prime number among these numbers. Find the sum of the digits of N.
EASY
Mathematics>Arithmetic>Playing with Numbers>Prime and Composite Numbers
Express 120 as a product of its prime factors.
EASY
Mathematics>Arithmetic>Playing with Numbers>Prime and Composite Numbers
The number of prime numbers which are less than 100 is:
HARD
Mathematics>Arithmetic>Playing with Numbers>Prime and Composite Numbers
For a,b∈0,1,⋯,9, a two digit number ab is defined for 10a+b. A two digit number ab is called a special prime if both ab and ba are primes. Then the total number of special primes is
EASY
Mathematics>Arithmetic>Playing with Numbers>Prime and Composite Numbers
Which of the following is not a pair of prime numbers ?
HARD
Mathematics>Arithmetic>Playing with Numbers>Prime and Composite Numbers
Suppose a,b,c are positive integers such that 2a+4b+8c=328. Then a+2b+3cabc is equal to-
EASY
Mathematics>Arithmetic>Playing with Numbers>Prime and Composite Numbers
If a and b are positive integers such that a2-b2 is a prime number, then