The GCD of two whole numbers is 5 and their LCM is 60. If one of the number is 20, then other number would be :

Options

A. 25
B. 13
C. 16
D. 15

Answer

To find the other number when one number is 20, and given that the GCD is 5 and the LCM is 60, we can use the relationship between GCD, LCM, and the two numbers.

### Step-by-Step Solution:

1. **Understand the relationship between GCD, LCM, and the two numbers**:
The relationship is given by the formula:
\[ \text{GCD}(a, b) \times \text{LCM}(a, b) = a \times b \]
where \( a \) and \( b \) are the two numbers.

2. **Assign the known values**:
Let \( a = 20 \) (the known number), and let \( b \) be the unknown number we need to find. We know:
- GCD = 5
- LCM = 60

3. **Substitute the known values into the formula**:
Using the formula:
\[ 5 \times 60 = 20 \times b \]

4. **Calculate the left side**:
\[ 5 \times 60 = 300 \]
So, we have:
\[ 300 = 20 \times b \]

5. **Solve for \( b \)**:
To find \( b \), divide both sides by 20:
\[ b = \frac{300}{20} = 15 \]

6. **Conclusion**:
The other number is \( 15 \).

### Final Answer:
The other number is **15**.