What is the fourth proportional to 3,7 and 15 ?

Options

A. 30
B. 45
C. 35
D. 25

Answer

To find the fourth proportional to the numbers 3, 7, and 15, we can set up a proportion based on the definition of fourth proportional.

### Step-by-Step Solution:

1. **Set Up the Proportion**:
We want to find a number \( x \) such that the ratio of 3 to 7 is the same as the ratio of 15 to \( x \). This can be expressed as:
\[ \frac{3}{7} = \frac{15}{x} \]

2. **Cross-Multiply**:
To eliminate the fractions, we can cross-multiply:
\[ 3 \cdot x = 7 \cdot 15 \]

3. **Calculate the Right Side**:
Now, calculate \( 7 \cdot 15 \):
\[ 7 \cdot 15 = 105 \]
So, we have:
\[ 3x = 105 \]

4. **Solve for \( x \)**:
Now, divide both sides by 3 to isolate \( x \):
\[ x = \frac{105}{3} = 35 \]

5. **Conclusion**:
Therefore, the fourth proportional to 3, 7, and 15 is:
\[ x = 35 \]

### Final Answer:
The fourth proportional to 3, 7, and 15 is **35**.