IMC 2015 Q8
8. How many two-digit squares differ by 1 from a multiple of 10?
- A 1
- B 2
- C 3
- D 4
- E 5
Intermediate Mathematical Challenge 2015, Question 8. © UK Mathematics Trust. Original paper on ukmt.org.uk
Answer: B 2
The two-digit squares are 16, 25, 36, 49, 64 and 81. Of these, only 49 and 81 differ by 1 from a multiple of 10.
Solution © UK Mathematics Trust, from the IMC 2015 solutions. Longer versions with investigations: Solutions and investigations (PDF).