Find the next number in the sequence

1, 2, 6, 21, 88, __?

I am a type of vehicle that has two wheels and is powered by humans. What am I?
Hint 1: You can ride me for exercise.
Hint 2: I am environment-friendly.

Can you place three balls such that the equation shown in the picture holds true?

If you're 8 feet away from a door and with each move you advance half the distance to the door. How many moves will it take to reach the door?

There are three boxes on a table. One of the box contains Gold and the other two are empty. A printed message contains in each box. One of the message is true and the other two are lies.

The first box says "The Gold is not here".

The Second box says "The Gold is not here".

The Third box says "The Gold is in the Second box".

Which box has the Gold?

Two-person want to cross a river. Only one boat is present on the riverside that can carry only one person at a time.

However, they still manage to cross the river. How is it possible?

Find the number of squares in the below picture

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!!!

I know a nine-letter word having only one vowel.
Do you ?

Can you name some countries which do not have the alphabet 'A' in them?