Transform the word THINK into BRAIN while changing only one letter at a time in a manner that each of the word in the process is a real word!!!

On a certain day, John celebrated his birthday. Two days later, his older twin brother Jacob celebrated his birthday.

Is this even possible? How can it be?

Two guards were on duty outside a barracks. One faced up the road to watch for anyone approaching from the North. The other looked down the road to see if anyone approached from the South. Suddenly one of them said to the other, "Why are you smiling?"

How did he know his companion was smiling?

Three people check into a hotel. They pay $60 for the rent of the room. After they check-in, the manager realize that the rent for the room is $55. So, he gives $5 to the bellboy and asks him to give it to them. The bellboy thinks that it will be difficult for the three people to share $5 among them and seeking the personal benefit, he pockets $2 and gives the remaining $3 to them.

Now, each person paid $20 and got back $1. In this manner, each of them paid just $19 which totals to the amount of $57. The bellboy has $2 with him and adding them, we get $59. So where is the remaining $1?

Find the next number in the series below

21 32 54 87 131 ?

Considering the below equation:

V + II - VI = 10
IV - X + VIII = 20
IV - VII + VI = 30

Then, I + II + III = ?

A girl was sitting in her hotel room when she heard a knock on the door. She opened the door and found that a man was standing outside.
The man said, "Oh! I am really sorry, I thought this was my room."
He then walked through the corridor to the elevator. The girl did not know the man. She closed her door and called security asking them to apprehend the man.
What made her suspicious of that man? He might have been genuinely mistaken.

A game is being played where eight players can last for thirty-five minutes. Six substitutes alternate with each player in this game. Thus, all players are on the pitch for the same amount of time including the substitutes.

For how long is each player on the pitch?

Replace the question mark with the correct number.

2 + 2= 4
3 + 3= 18
4 + 4= 48
5 + 5= ?

As shown in the image, the nine Dogs are square fenced. By constructing just two square fences can you make sure that two Dogs cannot meet each other without crossing the fence?