24885

First, I decided to try multiplying 3.125 by 2, then 3, then 4, then 5, etc: However, after finding out that 3.125 multiplied by 2 was 6.250, I realized that 0.250 was basically a quarter. Therefore, I worked out that I needed to multiply by two and then four, so in other words multiply by 8, to get an answer. When I had multiplied 3.125 by 8, I found out that the answer was 25.

The sums I completed:
3.125 X 2 = 6.250
3.125 X 8 = 25