Arithmetic Course/Number System

Number of Base Ten
The number system in use of the ordinary real life is number in of base ten consists of digit from 0 to 9. Any number greater than 9 can be expressed as
 * $$A \times 10^n + B \times 10^{(n-1)} + \cdots + Z \times 10^1 + 10^0$$

For example
 * $$121 = 1 \times 10^2 + 2 \times 10^1 + 1 \times 10^0 = 100 + 20 + 1$$

Number of Base Two
The number system in use of the ordinary real life is number in of base ten consists of digit from 0 to 1. Any number greater than 1 can be expressed as
 * $$A \times 2^n + B \times 2^{(n-1)} + \cdots + Z \times 2^1 + 2^0$$

For example
 * $$111_2 = 1 \times 2^2 + 2 \times 2^1 + 1 \times 2^0 = 4 + 2 + 1 = 7$$