The decimal system is a system of tens, which uses ten digits (0, 1, 2, 3, 4, 5, 6, 7, 8 and 9).
Each digit has a place value, depending on its position in the number.
For example, 5968 is a four-digit number, with 5 being the first or thousands digit, 9 being the second or hundreds digit, 6 being the third or tens digit, and 8 being the fourth (last) or units digit. So, 5968 = (5 × 1000) + (9 × 100) + (6 × 10) + (8 × 1).

Example
Find a three-digit number whose first digit is 1. The second digit is twice the last, and the difference between the first two digits is 7. Solution.
The first digit is given as 1.
Since the difference between the first two digits is 7, the second digit must be 8.
Since the second digit is twice the last, the last digit is 4.
Thus, the required three-digit number is 184.