The fourth proportional of 3, 7, and 15 is____.

Options

A. 35
B. 15
C. 7
D. 21

Explanation

To find the fourth proportional of the numbers 3, 7, and 15, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Concept of Fourth Proportional**:
The fourth proportional of three numbers \(a\), \(b\), and \(c\) is a number \(x\) such that the ratio \(a:b = c:x\).

2. **Set Up the Equation**:
Given the numbers are 3, 7, and 15, we can set up the equation as follows:
\[ \frac{3}{7} = \frac{15}{x} \]

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

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

5. **Solve for \(x\)**:
To find \(x\), divide both sides by 3:
\[ x = \frac{105}{3} \]

6. **Perform the Division**:
Now, calculate \(105 \div 3\):
\[ x = 35 \]

7. **Conclusion**:
Therefore, the fourth proportional of 3, 7, and 15 is:
\[ \boxed{35} \]