Find the average of the first four prime numbers greater than 10

Analyze the question: We need the first four prime numbers greater than 10, then find their average (mean).

Concept: A prime number has exactly two factors: 1 and itself. Average = (sum of the numbers) ÷ (how many numbers there are).

Solution:

Check the numbers after 10:

• 11 is prime
• 12 is not prime (even)
• 13 is prime
• 14 is not prime (even)
• 15 is not prime (3 × 5)
• 16 is not prime (even)
• 17 is prime
• 18 is not prime (even)
• 19 is prime

The first four primes greater than 10 are 11, 13, 17, 19.

Sum = 11 + 13 + 17 + 19 = 60

Average = 60 ÷ 4 = 15

Answer: The average is 15, which is option D.

Tip: In prime-number questions, skip all even numbers (except 2) and numbers ending in 5 right away. This saves time and reduces errors. Also, check your sum by pairing: 11 + 19 = 30 and 13 + 17 = 30, so the total is 60. Pairing numbers that add up to round figures makes mental arithmetic much faster in the exam hall.