What's the only room from which no one can enter or leave

I’m tall when I’m young, and I’m short when I’m old. What am I?

Which digit occurred maximum times between the number 1 and 1000?

Forwards I am heavy, backward I am not. What am I?

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

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There is a unique number which when multiplied by any number from 1 to 6, we will get the new number that contains the same digits only.

Can you find that number?

A tree doubled in height each year until it reached its maximum height over the course of ten years. How many years did it take for the tree to reach half its maximum height?

You have two buckets - one holds exactly 5 gallons and the other 3 gallons. How can you measure 4 gallons of water into the 5 gallon bucket?

(Assume you have an unlimited supply of water and that there are no measurement markings of any kind on the buckets.)

In a box, there is a jumble of 7 red balls, 6 blue balls, 5 green balls, and 4 yellow balls. What is the minimum number of balls, will you have to pick up so that you have at least 4 balls of the same colour?

You need to arrange three 7 and mathematical symbols to form number 1?