Solution:

It is mentioned that 'all but fifteen pencils' were used by him. This phrase means that except fifteen, all his pencils were used.

Thus, he is left with fifteen pencils.

Solution:

No

Solution:

Since every bacterium divides into two in two minutes and we know the fact that the dish was half filled with bacteria at 3 pm, after exactly 2 minutes, all will divide into two and will fill the petri dish. Thus, the petri dish will become full at 3:02 pm.

Solution:

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

Solution:

Chalk Board

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:

I am a secret.

Solution:

Chicago

Solution:

A green paint off-course :p