To find out how much the fee to join the program was, let's set up the problem with the information given.
- Let \( F \) represent the fee to join the program.
- The cost to try each animal diet is $15.
- Justin tried three animal diets: lion, flamingo, and panda. Therefore, the cost for the diets alone is \( 3 \times 15 = 45 \) dollars.
Now we can write the equation for the total amount Justin paid:
\[ F + 45 = 66 \]
To find \( F \), we subtract 45 from both sides:
\[ F = 66 - 45 = 21 \]
Thus, the fee to join the program was $21.
The answer is $21.