Find a 9-digit number, which you will gradually round off starting with units, then tenth, hundred etc., until you get to the last numeral, which you do not round off. The rounding alternates (up, down, up ...). After rounding off 8 times, the final number is 500000000. The original number is commensurable by 6 and 7, all the numbers from 1 to 9 are used, and after rounding four times the sum of the not rounded numerals equals 24.

A word I know,
Six letters contain,
Subtract just one,
And twelve is what remains.

In Greek mythology, the Sphinx sat outside of Thebes and asked this riddle of all travellers who passed by. If the traveller failed to solve the riddle, then the Sphinx killed him/her. And if the traveller answered the riddle correctly, then the Sphinx would destroy herself. The riddle:

What goes on four legs in the morning, on two legs at noon, and on three legs in the evening?

Oedipus solved the riddle, and the Sphinx destroyed herself.

The more it dries, the wetter it becomes. Can you tell what it is?

What do you get if you add 2 to 100 four times?

What occurs twice in a week, once in a year but never in a day?

What time is it when an elephant sits on a fence?

What is the fewest number of coins that would be required in order to make sure each and every coin touches exactly three other coins?

John can fit six large chocolate boxes or nine small chocolate boxes into a carton. How many cartons will he require to put sixty-six chocolate boxes into?

Complete the following series. It.s very logical:

1, 4, 5, 6, 7, 9, 11, __?