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

All of Jacob's shoes are red except two.
All of Jacob's shoes are green except two.
All of Jacob's shoes are yellow except two.

How many shoes does Jacob have?

What is next number in the pattern below:

131 517 192 123 ?

It has five wheels, though often think four, You cannot use it without that one more, You can put things in it, you can strap things on top, You can't find it in the market, but you can still go shop. What is it?

What is the centre of gravity?

Replace the question mark with the correct number in below Picture:

Yesterday I fell from 30 feet high ladder but I don't get hurt. Why?

Look at this sequence from top to bottom. What is the next number in the sequence?
1
11
21
1211
111221
312211

People are waiting in line to board a 100-seat aeroplane. Steve is the first person in the line. He gets on the plane but suddenly can't remember what his seat number is, so he picks a seat at random. After that, each person who gets on the plane sits in their assigned seat if it's available, otherwise, they will choose an open seat at random to sit in. The flight is full and you are last in line. What is the probability that you get to sit in your assigned seat?

Can you solve this amazing hard number series problem?
7, 8, 10, 12, 16, 18 ?