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!