HKIMO 2017 Grade 5 National Final (Heat) Set 04 Question 5

A bag contains 8 blue balls, 7 red balls and 9 green balls. At least how many must be picked to guarantee 2 balls of each color?

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