Answer: Secret Code #288

The secret message says:

MOTOR

Decode it with Python

This program does the decoding for you. Change the secret and the code to crack other messages!

secret = "13-15-20-15-18"
letters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
words = []

for word in secret.split(" / "):
    decoded = ""
    for number in word.split("-"):
        decoded = decoded + letters[int(number) - 1]
    words.append(decoded)

print(" ".join(words))

When you run it, it prints:

MOTOR