Find out the missing number in the following sequence:
$1, 3, 3, 6, 7, 9, ?, 12, 21$

In the set of three-digit numbers given below,the first two digits of each number are interchanged. Which one among them will be in the middle if they are arranged in ascending order after the change?
$230, 422, 328, 516, 138$

The following questions are based on the five three-digit numbers given below:
$972, 526, 487, 359, 251$
If $1$ is subtracted from the last digit of each of the above numbers,the sum of the digits of how many of them are prime numbers?

Fill in the blank space for the given series with the correct alternative.
In the series $2, 6, 18, 54, \ldots,$ what will be the $8^{th}$ term?

The positions of how many digits in the number $7,3,5,4,6,1,2$ will remain unchanged after the digits within the number are rearranged in descending order from left to right?

The following question is based on the five three-digit numbers given below:
$519, 368, 437, 246, 175$
Which of the following is the third digit of the second highest number?