Using mathematics and the deposit or money multiplier, explain “money expansion by the banking system?

Answer 1

Check out:
http://en.wikipedia.org/wiki/Fractional_Reserve_Banking

The bottom line is that a bank can lend out the $1 you deposit multiple times so more than one person thinks s/he can spend that money.

Just how much money can be created from your dollar is determined by the reserve requirements established by the Federal Reserve.