HKIMO 2018 Grade 5 National Final (Heat) Set 03 Question 5

A bag contains 10 blue balls, 18 red balls and 5 green balls. At least how many balls must be drawn at random to guarantee 3 balls of each color?

Source glossary (English–Vietnamese): At least: ít nhất; Ensure: chắc chắn; Each colour: mỗi màu.