Can you find a pattern and the missing number in the series given below?

100, 155, 258,? , 584, 819.

I came first on earth but second on the heaven.
I also came twice in a week but found just once in a year.
I stay away from months but you can find me in February.

What am I?

I have branches, but no fruit, trunk or leaves. What am I?

What's a single-digit number with no value?

There is one thing that goes round the house and also inside the house but never even share a brief touch. What is it?

I have an eye but still cannot see.

Who am I?

Andrew’s doctor gives him three pills and tells him to take one every half hour. How much time will have passed by the time Andrew’s taken all three pills?

A woman lives in a skyscraper thirty-six floors high and is served by several elevators which stop at each floor going up and down. Each morning she leaves her apartment and goes to one of the elevators. Whichever one she takes is three times more likely to be going up than down. Why?

What can you see in the middle of March and April that you can never see in any other month?

Twice ten are six of us,

Six are but three of us,

Nine are but four of us;

What can we possibly be?

Would you know more of us?

Twelve are but six of us,

Five are but four, do you see?

What are we?