Institution: Bangladesh Bank Exam: Officer (General) |-Sonali | Janata | Bangladesh Krishi_Bangladesh Development bank ২৪.০৫.২০১৯ ||
Topic: Arithmetic Mathematics (পাটিগণিত), Subject: Mathematics (সাধারণ গণিত)

Best Answer Explained in Below-:

To solve the problem, we first need to determine the current amount of water in the 20 kg mixture. Since the mixture contains 10% water, we can calculate the amount of water as follows:

\[
\text{Water} = 10\% \text{ of } 20 \text{ kg} = 0.10 \times 20 = 2 \text{ kg}
\]

This means there are 2 kg of water and 18 kg of spirit in the mixture. Next, let’s denote the amount of water we need to add as \( x \) kg. After adding \( x \) kg of water, the new total weight of the mixture becomes:

\[
20 \text{ kg} + x \text{ kg} = 20 + x \text{ kg}
\]

The new amount of water will be:

\[
2 \text{ kg} + x \text{ kg}
\]

We want the percentage of water in the mixture to reach 25%, which can be expressed with the equation:

\[
\frac{2 + x}{20 + x} = 0.25
\]

Cross-multiplying gives us:

\[
2 + x = 0.25(20 + x)
\]

Expanding the right side:

\[
2 + x = 5 + 0.25x
\]

Rearranging the equation:

\[
x - 0.25x = 5 - 2
\]

\[
0.75x = 3
\]

Solving for \( x \):

\[
x = \frac{3}{0.75} = 4
\]

Thus, to raise the percentage of water to 25%, 4 kg of water must be added. Therefore, the correct answer is option B: 4.