21
35
31
39
Answer is Wrong!
Answer is Right!
The correct answer is (c).
The sum of the 7 numbers is $7 \times 25 = 175$. The sum of the first 3 numbers is $3 \times 20 = 60$, and the sum of the last 3 numbers is $3 \times 31 = 93$. Therefore, the remaining number is $175 – 60 – 93 = 31$.
Option (a) is incorrect because $21$ is not the sum of any three of the numbers. Option (b) is incorrect because $35$ is not the sum of any four of the numbers. Option (d) is incorrect because $39$ is not the sum of any five of the numbers.