We can use the formula:
Simple Interest (SI) = (P * R * T) / 100
Where P is the principal amount, R is the rate of interest per annum, and T is the time duration for which the amount was borrowed.
Let's substitute the given values:
SI = 15,250 - 10,000 = 5,250
P = 10,000
R = 7.5%
Now, we can rearrange the formula to find T:
T = (SI * 100) / (P * R)
Substituting the values:
T = (5,250 * 100) / (10,000 * 7.5)
T = 7 years
Therefore, the farmer used the sum for 7 years.