Question

${21}^2-1$ is a product of two consecutive even numbers. Find those numbers.

• A. 21 and 22
• B. 22 and 24
• C. 20 and 22
• D. 22 and 23

Answer 1

Answer: (C)

20 and 22

${21}^2-1=400$
We can express the above number into general form $a^2-1=(a+1)\times (a-1)$
Where $a=21$
So, ${21}^2-1=(21+1)\times (21-1)$ $=22\times 20=400$
Therefore, the two even consecutive numbers are 20 and 22.