What's the only room from which no one can enter or leave

If,
29 - 1 = 30
9 - 1 = 10
14 - 1 = 15

Based on similar logic, Can you prove that the below algebraic equation is true?
11 - 1 = 10 ?

What is the centre of gravity?

I make a loud sound when I’m changing. When I do change, I get bigger but weigh less. What am I?

I am a type of room you cannot enter or leave. Raised from the ground below, I could be poisonous or a delicious treat. What am I?

The day before yesterday I was 25 and the next year I will be 28. This is true only one day in a year. What day is my birthday?

A B C D E F G H

These are the letters given to you. Now you have to find out the letter that comes two to the right of the letter which is immediately to the left of the letter that comes three to the right of the letter that comes midway between the letter two to the left of the letter C and the letter immediately to the right of the letter F.

There are are five things wrong with this sentence; only geniuses will be able to to spot all of the mitstakes

Can you solve the below algebraic mathematical equation?

(J+O+I+N+T)3 = JOINT

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?