Institution: Bangladesh Development Bank Exam: Bangladesh Development Bank Ltd - Officer - 28.03.2014
Topic: Mathematics (সাধারণ গণিত), Subject: Mathematics (সাধারণ গণিত)

Best Answer Explained in Below-:

To understand the difference between simple and compound interest, we first need to calculate both types of interest on a principal sum of Tk.1000 at a rate of 10% per annum over 4 years.

**Simple Interest (SI)** is calculated using the formula:
\[ \text{SI} = P \times r \times t \]
Where:
- \( P \) is the principal amount (Tk. 1000)
- \( r \) is the rate of interest (10% or 0.10)
- \( t \) is the time in years (4 years)

Calculating simple interest:
\[ \text{SI} = 1000 \times 0.10 \times 4 = 400 \]
So, the total amount with simple interest after 4 years is:
\[ \text{Total SI Amount} = 1000 + 400 = 1400 \]

**Compound Interest (CI)**, on the other hand, is calculated using the formula:
\[ \text{CI} = P \times (1 + r)^t - P \]
Using the same values:
\[ \text{CI} = 1000 \times (1 + 0.10)^4 - 1000 \]

First calculate \( (1 + 0.10)^4 \):
\[ (1.10)^4 = 1.4641 \]
Now calculate the compound interest:
\[ \text{CI} = 1000 \times 1.4641 - 1000 = 1464.1 - 1000 = 464.1 \]

Finally, the difference between compound and simple interest is:
\[ \text{Difference} = \text{CI} - \text{SI} = 464.1 - 400 = 64.1 \]

Thus, the difference between simple and compound interest on Tk.1000 at 10% over 4 years is Tk.64.10, making option D the correct answer.