This is a simple decoding puzzle. And will set you for some more interesting ones in the future. The test you can have for yourself, is how *fast* can you solve this.
The encoding / decoding has to work only for digits. 0 (zero) to 9 (nine). The first input will be a string of 10 characters. This encoding key will define what the mapping with 0-9. The next will be a mathematical operation that you have to carry out. The equation will be in encoded format, you have to decode and provide the answer.
Consider the example:
Input 1:
2345678901
Input 2:
12 + 23
Answer:
91 (90 + 01)
See if your code can work even with characters and symbols as the encoding key!