I am a seven-letter word but if you remove four of my letters I will have one left. What word am I?

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.

Why Aeroplane windows are round?

Count the number of lines in the image below ?

If in a car race, the man who came two places in front of the last man finished one ahead of the man who came fifth, how many contestants were there?

Find the next number in the series

12, 20, 40, 72, 116, 172 ?

Assume there are approximately 5,000,000,000 (5 billion) people on Earth. What would you estimate to be the result, if you multiply together the number of fingers on every person's left hand? (For the purposes of this exercise, thumbs count as fingers, for five fingers per hand.) If you cannot estimate the number then try to guess how long the number would be.

I am looking for a word that change both gender and number when i add the letter 'S'.

What Word am I Looking For ?

It comprises roots which nobody sees,
Much taller than those trees,
Up and up it goes,
Still it never grows?

If you toss a coin 10 times and it lands heads up every time, what are the chances it will land heads up if you toss it again?