Chapter 4.3, Problem 40E

Decoding Messages If we assign numbers to the letters of the alphabet as follows and assign 27 to a blank space, we can convert a message to a numerical sequence. We can “encode” a message by adding 3 to each number that represents a letter in a message.

Thus, the message “Go for it” can be encoded by using the numbers to represent the letters and further encoded by using the function C(x) = x + 3. The coded message would be 10 18 30 9 18 21 30 12 23. Find the inverse of the function and use it to decode 23 11 8 30 21 8 4 15 30 23 11 12 17 10.