Subject: General Aptitude · Chapter: Number System · Exam: · Marks: · Difficulty:
In the following sum, '?' stands for which digit? ? + 1? + 2? +? 3 +? 1 = 21?
(a)4
(b)6
(c)8
(d)9
(e)None of these
Answer
Answer (as printed): C
Explanation
Let the required digit be x. Then x + 1x + 2x + x3 + x1 = 21x ⇒ x + 10 + x + 20 + x + 10x + 3 + 10x + 1 = 200 + 10 + x ⇒ 22x = 210 – 34 = 176 ⇒ x = 8. Hence, the required digit is 8.
Explanation as extracted from the printed page; notation may be imperfect.