900 Numbers
Source: 1993 National High School Mathematics League, Exam One, Problem 12
February 28, 2020
Problem Statement
There are 900 3-digit-numbers . Print them on 900 cards. Some numbers are still numbers when we turn upside down, for example is when we turn upside down. However, some numbers are not, for example . So, some numbers cam be used twice. If we want to express all 3-digit-numbers, we can print________cards fewer.