Multiple choice general knowledge math & puzzles A number added to twice itself gives twenty-one. Find that number! 8 7 3 9 Reveal answer Fill a bubble to check yourself B Correct answer Explanation Let the number be x. The problem states x + 2x = 21. Combining like terms: 3x = 21. Dividing both sides by 3: x = 7. Checking: 7 + 2(7) = 7 + 14 = 21.