Answer:
680
Step-by-step explanation:
Use the binomial coefficient where you choose numbers out of possible numbers and find the total amount of combinations since order does not matter:
Thus, you can make 680 three-non-repeating-number codes
If x∆y = 3x - y², then
5∆1 = 3×5 - 1² = 15 - 1 = 14
and
14∆6 = 3×14 - 6² = 42 - 36 = 6
So (5∆1)∆6 = 6.