In a class of 40 pupils, the number of girls is three-fifths of the number of boys. Find the number of boys in the class.

Answer

To solve the problem, we need to find the number of boys in a class of 40 pupils, where the number of girls is three-fifths of the number of boys. Let's break this down step by step.

### Step-by-Step Solution

1. **Define Variables:**
Let the number of boys be \( y \) and the number of girls be \( x \).

2. **Set Up the Equations:**
According to the problem, the total number of pupils in the class is 40. Therefore, we can write the equation:
\[ x + y = 40 \]

3. **Express Girls in Terms of Boys:**
The problem states that the number of girls is three-fifths of the number of boys. This can be expressed as:
\[ x = \frac{3}{5}y \]

4. **Substitute the Expression for Girls:**
Now, we can substitute the expression for \( x \) from the second equation into the first equation:
\[ \frac{3}{5}y + y = 40 \]

5. **Combine Like Terms:**
To combine the terms on the left side, we need a common denominator. The equation becomes:
\[ \frac{3}{5}y + \frac{5}{5}y = 40 \]
This simplifies to:
\[ \frac{8}{5}y = 40 \]

6. **Solve for \( y \):**
To isolate \( y \), multiply both sides of the equation by \( \frac{5}{8} \):
\[ y = 40 \times \frac{5}{8} \]
Simplifying this gives:
\[ y = \frac{200}{8} = 25 \]

7. **Conclusion:**
Therefore, the number of boys in the class is \( 25 \).

### Final Answer:
The number of boys in the class is **25**.