Solution:

50%

Explanation:
The probability will be 50% only as it always was. The next flip of coin is not dependent on what the results were in the previous flips.

Solution:

The clerk said that every day the password is CHANGED. Changed is the password.

Solution:

Chalk Board

Solution:

25%

Suppose every item is priced at $100. On 20% discount, the price will become $80. Now he must add $20 to each item for original price which is 25% of $80.

Solution:

What

Solution:

See o double you

Solution:

Birbal drew another line that was larger than the pre-drawn line. Now the pre drawn line was smaller.

Solution:

2, 3, 10 and 10
To maintain the difference of eight and keeping in mind the third and fourth hint, the first and third cards can be 2 and 10 or 10 and 2. But keeping in mind the first hint, we know that the first card is 2 and the third is 10. Similarly, if we see the fifth hint, the difference between the second and fourth card should be 7. Thus we know that the second card is 3 and the fourth card is 10.

Solution:

I am a secret.

Solution:

Chicago