Broom.

Making love with the mother.

A mother is 21 years older than her son.
Let the son be S years old
Then the mother is S+21

6 years from now exactly, the mother will be exactly 5 times as old as the child.

S+21+6 = 5(S+6)
S+27 = 5S+30
-3 = 4S
-3/4 = S

So the son is negative three-fourths of a year old, which means he has not been born yet. Since three fourths of a year before he is born is 9 months before he is born, he is just being conceived.

A bottle.

Robert is a dog and his human equivalent age is calculated as follows:
human equivalent age = 16 x ln (dog's chronological age) + 31

ln is natural logarithm

He has 2 identical books, and in order to increase the value of one of them, he burned the second one.

A promise.

Jonny.

B, D and E are the liars.
First, if A was a liar, then C would be a liar. But then D would say the truth, which contradicts A’s statement about B lying. Therefore, A says the truth. That means C also says the truth. You can use logic to prove that this doesn’t make a contradiction.

You can say any sentence with a true fact, but the subject of the fact must be vaguely named. For example: “A scientist discovered that things were able to fall to another object due to gravity.” In this sentence, it’s obviously about how Isaac Newton discovered gravity, BUT his name wasn’t ‘A scientist.’ Sure, it’s a grey area, but it still works.

One of the ten people had the key to the room. He unlocked the door, except there was nobody outside of the room.

20%.
After the first cut, the cake is split into a 40% piece and a 60% piece. The 60% piece is then cut into four 15% pieces, so 40% + 15% + 15% makes 70% of the cake. Then, the 40% piece from the first cut is split into two 20% pieces.

2.

11 o'clock plus 2 hours = 1 o'clock 9 o'clock plus 5 hours = 2 o'clock