10+10 and 11+11 can give you the same answer.

Explain how?

Find The Next Number In The Sequence

30 10 15 13 0 16 -15 ?

There are 2 people near the river and both of them want to get on the other side. The boat can only take one of them. But they got across.
How?

If you were to put a coin into an empty bottle and then insert a cork into the neck, how could you remove the coin without taking out the cork or breaking the bottle?

Spot the cat in the picture below

There is a shop where written:
Buy 1 for $1
10 for $2
100 for $3
I needed 999 and still only paid $3. How could this be financially viable for the shop-keeper?

In 1990, a person is 15 years old. In 1995, that same person is 10 years old. How can this be?

In the addition below, all digits have been replaced by letters. Equal letters represent equal digits and different letters represent different digits.
ABCABA
BBDCAA
ABEABB
ABDBAA
------- +
AAFGBDH
What does the complete addition look like in digits?

A high school has a strange principal. On the first day, he has his students perform an odd opening day ceremony:

There are one thousand lockers and one thousand students in the school. The principal asks the first student to go to every locker and open it. Then he has the second student go to every second locker and close it. The third goes to every third locker and, if it is closed, he opens it, and if it is open, he closes it. The fourth student does this to every fourth locker, and so on. After the process is completed with the thousandth student, how many lockers are open?

I look at you, you look at me, I raise my right, you raise your left. What am I?