IMC 2018 Q2
2. How many of these four integers are prime?
1 11 111 1111
- A 0
- B 1
- C 2
- D 3
- E 4
Intermediate Mathematical Challenge 2018, Question 2. © UK Mathematics Trust. Original paper on ukmt.org.uk
Answer: B 1
First note that 1 is not a prime. Of the other integers, 11 is prime; 111 = 3 × 37, so is not prime; 1111 = 11 × 101, so is not prime.
Solution © UK Mathematics Trust, from the IMC 2018 solutions. Longer versions with investigations: Solutions and investigations (PDF).