To find the radius of the original circle, you can use the scale factor. The formula to calculate the original radius when you know the scaled radius and the scale factor is:
\[ \text{Original Radius} = \text{Scaled Radius} \div \text{Scale Factor} \]
In this case, the scaled radius is 18 cm, and the scale factor is \( \frac{1}{5} \).
Substituting the values into the formula:
\[ \text{Original Radius} = 18 , \text{cm} \div \left(\frac{1}{5}\right) = 18 , \text{cm} \times 5 = 90 , \text{cm} \]
Thus, the radius of the original circle is 90 cm.