Find the next number in the sequence

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

What 4-letter word can be written forward, backward or upside down, and can still be read from left to right?

How many people must be gathered together in a room, before you can be certain that there is a greater than 50/50 chance that at least two of them have the same birthday?

Which five letter word can be added at the end of below word to form a
valid word?

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I look at you, you look at me, I raise my right, you raise your left. What am I?

What is more useful when it is broken?

All of the flowers I have are orchids except two. All of the flowers I have are hibiscuses except two. All of the flowers I have are roses except two.

Can you tell me how many flowers I have?

You have three orange, two pink and five purple balls in the drawer beside your bed. There is no electricity and the room is entirely dark. How many balls must you take out to ensure at least one ball of each colour at least?

The person who makes it has no need for it; the person who buys it has no use for it. The person who uses it can neither see nor feel it. What is it?

There are a number of books on a shelf. If one book is the 4th from the left and 6th from the right, how many books are on the shelf?