Lennon used the property o addition
There's 9 choices for the first digit and 4 choices for the last digit. The number of choices for the 2nd and 3rd digits is 90 if 2 numbers are different. or 100 if duplicates are allowed.
If duplicates are allowed the answer is 9 * 100 * 4 = 3600 possible numbers.
Answer:
Plot the coordinates on a graph and join them to form a triangle. After doing that, you can count the length of each triangular side and find the sum of the 3 sides. That will be your answer.