Find the 4th proportional to 3,7 and 9.

Answer

To find the 4th proportional to the numbers 3, 7, and 9, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the concept of proportion**:
In a proportion, if we have four quantities A, B, C, and D, they are in proportion if:
\[ \frac{A}{B} = \frac{C}{D} \]
This can be rearranged to:
\[ A \cdot D = B \cdot C \]

2. **Assign the values**:
For this problem, we assign:
- \( A = 3 \)
- \( B = 7 \)
- \( C = 9 \)
- \( D = ? \) (This is what we need to find)

3. **Set up the equation**:
According to the proportion, we can write:
\[ 3 \cdot D = 7 \cdot 9 \]

4. **Calculate \( 7 \cdot 9 \)**:
First, calculate \( 7 \cdot 9 \):
\[ 7 \cdot 9 = 63 \]

5. **Substitute back into the equation**:
Now we have:
\[ 3 \cdot D = 63 \]

6. **Solve for \( D \)**:
To find \( D \), divide both sides by 3:
\[ D = \frac{63}{3} = 21 \]

7. **Conclusion**:
Thus, the 4th proportional to 3, 7, and 9 is:
\[ D = 21 \]

### Final Answer:
The 4th proportional to 3, 7, and 9 is **21**.

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