When I get multiplied by any number, the sum of the figures in the product is always me. What am I?

I am taken from a mine and shut up in a wooden case, from which I am never released, and yet I am used by almost every person. What am I?

I am a seven-letter word but if you remove four of my letters I will have one left. What word am I?

I am white when I am dirty, and black when I am clean. What am I?

Count the number of times the letter "F" appears in the following paragraph:

FAY FRIED FIFTY POUNDS OF
SALTED FISH AND THREE POUNDS
OF DRY FENNEL FOR DINNER FOR
FORTY MEMBERS OF HER FATHER'S FAMILY.

Why 1729 is a magic number? Answer Logically.

If you say my name, I will no longer exist. What am I?

During which month do people sleep the least?

I have a head like a cat and feet like a cat, but I am not a cat. What am I?

When my father was 31 I was 8. Now he is twice as old as me. How old am I?