The differences between the consecutive terms are perfect squares. Subtracting the second term from the first gives 125 - 80 = 45, which is 7 squared; subtracting the third term from the second gives 80 - 45 = 35, which is not a perfect square. Recalculating the differences properly: 125 minus 80 is 45, 80 minus 45 is 35, and 45 minus 20 is 25; the differences of these first differences are 10, 10, showing a pattern where the numbers themselves relate to subtraction of squares (125 - 45 = 80 is 6 squared + 9, but actually 125 - 80 is 45, 80 - 45 is 35, 45 - 20 is 25, meaning the subtractions are decreasing by 10). Let's use the correct pattern: 125 minus 5 squared is 100, wait, 125 minus 80 is 45, 80 minus 45 is 35, 45 minus 20 is 25. The next difference must be 15, so 20 minus 15 is 5.