Find the wrong number in the series 121, 169, 289, 400, 529

Look for a common rule among the numbers.

• 121 = 11², and 11 is prime.

• 169 = 13², and 13 is prime.

• 289 = 17², and 17 is prime.

• 400 = 20², but 20 is not prime.

• 529 = 23², and 23 is prime.

So all numbers except 400 are perfect squares of prime numbers. Therefore, 400 is the wrong number in the series.