Solution:

43

Let us break the problem into different parts.
We know that after six, all the numbers that are divisible by three can be ordered. This is because we can expand them as the sum off sixes and nines.
After twenty six, the numbers that are divisible by three if subtracted by twenty can also be ordered.
After forty six, all the numbers will fit in one of the already discussed category. Thus all the numbers can be ordered.
So forty three is the last number that does not come under any of the already discussed category.
44 = 20 + 6 * 4, 45 = 6 * 6 + 9

Solution:

I + IX = X

The equation read from the other side becomes X = XI + I which is false.

Solution:

They used the planks in the manner that is shown in the figure.

Solution:

4

No Of circles in the number.

Solution:

When you were inside the shed, the murderer replaced the lock with his own one that was identical to your original lock. When you locked the store room, you did not require a key and there was nothing abnormal for you. When you left, the murderer opened the lock with his key, planted the dead body inside and replaced the lock again putting the original lock in the place. He then closed it without any problem.

Solution:

This statement is true but only in a specific case i.e. if both of the siblings continue to live till the day the younger is as old as the elder one was during the time of their birth.

Solution:

The probability of two numbers being relatively prime is 60%. Thus if you keep playing the bet, you will hypothetically make 20 cents on every play.

Thus, you should accept the bet.

Solution:

192

Explanation:
I started with the number 192.
Adding the reversed 291 to 192 gave me 483.
Similarly,
483 + 384 = 867
867 + 768 = 1635
1635 + 5361 = 6996

Solution:

The Password.

Explanation:
Clues 2 and 5 are correct and rest are lies.

Solution:

The check was for dollars 31.63.

The bank teller gave her dollars 63.31
She spent .05, and then she had dollars 63.26, which is twice the check.

Let x be the dollars of the check, and y be the cent.

The check was for 100x + y cent
He was given 100y + x cent

Also
100y + x - 5 = 2(100x + y)
Expanding this out and rearranging, we find:
98y = 199x + 5

or 199x ≡ -5 (mod 98)
or 98*2*x + 3x ≡ -5 (mod 98)
3x ≡ -5 ≡ 93 (mod 98)
this quickly leads to x = 31

Solution:

   9567
+ 1085
======
 10652