**Calculating Future Value**

To calculate the future value of an amount, we use the formula:

Future Value = Present Value * (1 + Interest Rate)^Number of Periods

In this case, we have a present value of Rs 100, an interest rate of 5%, and a time period of 20 years.

**Step 1: Calculate the Interest Rate**

First, we need to convert the interest rate from a percentage to a decimal. To do this, we divide the interest rate by 100:

Interest Rate = 5% / 100 = 0.05

**Step 2: Calculate the Future Value**

Using the formula mentioned above, we can now calculate the future value:

Future Value = Rs 100 * (1 + 0.05)^20

**Step 3: Simplify the Formula**

To simplify the formula, we need to calculate the value within the parentheses first:

(1 + 0.05)^20 = (1.05)^20

**Step 4: Calculate (1.05)^20**

Calculating (1.05)^20 can be done using a calculator or by using the concept of compound interest.

(1.05)^20 = 2.653297705

**Step 5: Calculate Future Value**

Now, we can substitute the value of (1.05)^20 back into the formula and calculate the future value:

Future Value = Rs 100 * 2.653297705 = Rs 265.33 (rounded to two decimal places)

Therefore, Rs 100 will become Rs 265.33 after 20 years at a 5% interest rate.

It is important to note that the calculation assumes a simple interest rate and does not take into account any additional factors such as compounding frequency or inflation. Additionally, the future value can vary depending on the specific terms and conditions of the investment or savings account.