Final answer:
The principal amount was calculated by subtracting the interest paid ($100) from the total repayment ($900), resulting in a principal amount of $800.
Step-by-step explanation:
To find the principal amount of the loan, we need to remove the interest paid from the total repayment amount. The total repayment was $900, which included a 10% interest, and we were told the interest amount was $100. We can therefore calculate the principal by subtracting the interest from the total repayment:
Principal = Total repayment - Interest
Principal = $900 - $100
Principal = $800
The principal amount of the loan was $800.