The centre of the third figure is empty. What digit should fit inside?

There’s a girl who has a large family. She has an equal amount of brothers and sisters, but each brother only has half as many brothers and sisters. What’s the correct amount of brothers and sisters?

An exterior architect is asked by a builder to plant seven trees in a manner that there are exactly six rows of trees in a straight line and each row has three trees in particular.

How will he do it?

1 + 9 + 8 = ?
considering, 28 + 8 + 92 = 10

How many letters are in the alphabet?

A thief is convicted in Mexico. He gets the death penalty. The judge allows him to say the last sentence to determine how the penalty will be carried out. If the thief lies, he will be hanged, if he speaks the truth he will be beheaded. The thief tells the last sentence and to everybody's surprise some minutes later he is set free because the judge cannot determine his penalty. What did the thief say?

What are the next two letters in the following series and why?
W A T N T L I T F S _ _

*Hint: Check Puzzle Title

Can you find out the remainder when 3^300 is divided by 5?

You may enter, but you may not come in. I have space, but no room. I have keys, but open no lock. What am I?

One is to three as three is to five and five is to four and four is the magic number.
What is the pattern?