Why is a river so rich ?

What does the below mathematical rebus mean?

Can anyone tell me a word in the English dictionary which has a silent 'z'?

You are trapped in a room with two doors. One leads to certain death and the other leads to freedom. You don't know which is which.
There are two robots guarding the doors. They will let you choose one door but upon doing so you must go through it.
You can, however, ask one robot one question. The problem is one robot always tells the truth, the other always lies and you don't know which is which.
What is the question you ask?

My grandson is about as many days as my son in weeks, and my grandson is as many months as I am in years. My grandson, my son and I together are 120 years. Can you tell me my age in years ?

If it took eight men ten hours to build a wall, how long would it take four men to build it?

Which alphabet letter is a question?

When is the top of a mountain similar to a savings account?

2, 3, 5, 9, 17, _ What is the next number in the sequence?

Find three numbers such that

* When we multiply three numbers, we will get the prime numbers.
* The difference between the second and the first number is equal to the third and second.