Prime Numbers Quiz

Find the total number of prime numbers less than 50.

• 13

• 15

• 17

• 14

Answer explanation

The prime numbers less than 50 are: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, and 47. Counting these gives a total of 15 prime numbers, making the correct answer 15.

What is the positive difference between the sum of all prime numbers between 11 and 20 (both included) and the sum of all prime numbers between 30 and 50 (both included)?

• 139

• 141

• 137

• 135

Answer explanation

The prime numbers between 11 and 20 are 11, 13, 17, 19, summing to 60. The prime numbers between 30 and 50 are 31, 37, 41, 43, 47, summing to 199. The positive difference is 199 - 60 = 139, which is the correct answer.

The greatest prime number less than 200 is:

• 199

• 193

• 197

• 191

Answer explanation

The greatest prime number less than 200 is 199. It is the largest number in the list of prime numbers below 200, which includes 191, 193, and 197, all of which are smaller than 199.

Which of the following numbers is prime?

• 323

• 571

• 513

• 715

Answer explanation

A prime number is only divisible by 1 and itself. 571 is prime as it has no divisors other than 1 and 571. The other numbers (323, 513, 715) can be divided by numbers other than 1 and themselves.

Find the smallest three digit prime number?

• 107

• 109

• 103

• 101

Answer explanation

The smallest three-digit prime number is 101. It is the first prime number that has three digits, as 100 is not prime (divisible by 2 and 5). The other options, 103, 107, and 109, are larger than 101.

Which of the following pairs of numbers are co-prime?

• 28, 81

• 12, 27

• 21, 56

• 36, 20

Answer explanation

The numbers 28 and 81 are co-prime because they have no common factors other than 1. In contrast, the other pairs share common factors, making them not co-prime.

One-third of the sum of all the prime numbers greater than 5 but less than 18 is the square of:

• 3

• 5

• 6

• 4

Answer explanation

The prime numbers greater than 5 and less than 18 are 7, 11, 13, and 17. Their sum is 48. One-third of 48 is 16, which is the square of 4. Thus, the correct answer is 4.

Which of the following is a prime number?

• 143

• 173

• 123

• 213

Answer explanation

A prime number is only divisible by 1 and itself. Among the choices, 173 is prime as it has no divisors other than 1 and 173. The other numbers (143, 123, 213) can be divided by numbers other than 1 and themselves.

Find the sum of prime no. between 50 and 60.

• 118

• 114

• 110

• 112

Answer explanation

The prime numbers between 50 and 60 are 53, 59. Their sum is 53 + 59 = 112. Therefore, the correct answer is 112.

Find the number of all prime numbers less than 55.

• 18

• 17

• 16

• 15

Answer explanation

The prime numbers less than 55 are: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53. Counting these gives a total of 16 prime numbers, making the correct answer 16.