Puzzle 3: Quickly solve and Quickly verify

1. There is a submarine that will only operate if there are exactly 20 passengers on board. If the first 19 seats were taken by average-sized men and the last seat was taken by a heavy woman, how come the submarine sank?

2.

3. Verify the correctness and apply the missing numbers in the below SUDOKU

4. The Fifth sister

5. The mind game, what can you see?

6. What is the missing number?