Calculation of Installment Amount:
The amount borrowed is ` 20,000 and it has to be repaid in annual installments of ` 2000 each. Therefore, the number of installments required to repay the loan is:

Number of Installments = Total Amount to be Repaid / Annual Installment Amount
Number of Installments = 20000 / 2000
Number of Installments = 10

Hence, Mr. Paul has to make 10 annual installments of ` 2000 each to repay the loan.

Calculation of Interest:
The loan is taken on the condition to repay with C.I. at 5% p.a. Therefore, the interest rate is 5% per annum. The interest for the first year can be calculated as:

Interest for the 1st year = (Principal Amount * Rate of Interest) / 100
Interest for the 1st year = (20000 * 5) / 100
Interest for the 1st year = 1000

The total amount to be repaid in the first year will be the sum of the annual installment and the interest for the first year, i.e.

Total Amount to be Repaid in the 1st year = Annual Installment Amount + Interest for the 1st year
Total Amount to be Repaid in the 1st year = 2000 + 1000
Total Amount to be Repaid in the 1st year = 3000

Calculation of Remaining Balance:
The remaining balance after the first year can be calculated as:

Remaining Balance after the 1st year = Principal Amount + Interest for the 1st year - Annual Installment Amount
Remaining Balance after the 1st year = 20000 + 1000 - 2000
Remaining Balance after the 1st year = 19000

This process can be repeated for the remaining years until the entire loan is repaid.

Calculation of Number of Years:
From the above calculations, we can see that Mr. Paul has to make 10 annual installments to repay the loan. Therefore, the number of years for the debt to be paid off is:

Number of Years = Number of Installments / 1 year
Number of Years = 10 / 1
Number of Years = 10

Hence, Mr. Paul has to repay the loan over a period of 10 years.