31. The tenth number is 10 + 1 = 11. The twentieth number is 20 + 1 = 21, therefore the thirtieth number is 30 + 1 = 31.

E. Each item is the first letter(s) of one of the seven continents:

1. Asia
2. North America
3. Africa
4. South America
5. Australia
6. Antarctica
7. Europe

K. The letters represent vitamins, and there is currently no Vitamin F, G, H, I or J.

2840.

This makes the first column the start of the Fibonacci series: 0, 1, 1, 2.
The second column is the even numbers: 2, 4, 6, 8
The third column is counting up by ones: 1, 2, 3, 4
And the last column is the answer in the first row in reverse: 8, 1, 2, 0.

Don’t ask what 50 × 2 equals. I have no idea. :)

Thanks to Jim for helping with the solution.