Replace the question mark with the number below given a simple math problem.

A woman is sitting in her hotel room and hears a knock at the door. She opens the door to see a man whom she’s never met before. He says, “I’m sorry, I have made a mistake, I thought this was my room.” He then goes down the corridor and into the elevator. The woman goes back into her room and calls security. What made the woman so suspicious of the man?

What is the last thing you take off before bed?

What is in seasons, seconds, centuries and minutes but not in decades, years or days?

A famous swimmer can swim downstream in a lake in exactly 40 minutes with the lake current.
He can swim upstream in that lake in exactly 60 minutes against the lake current.
The length of the lake is 2 km.

How long he can cover the distance of one side at a still lake with no current?

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?

You have a three-gallon and a five-gallon measuring device. You wish to measure out four gallons.

Can you count the number of blocks in the picture below?

Which letter of the alphabet has the most water?

What goes up but never comes down?