To find the radius of the original circle that was dilated using a scale factor of 1/5, you can use the formula:
\[ \text{Original Radius} = \frac{\text{Dilated Radius}}{\text{Scale Factor}} \]
In this case, the dilated radius is 18 cm and the scale factor is 1/5.
So, the calculation will be:
\[ \text{Original Radius} = \frac{18 , \text{cm}}{1/5} = 18 , \text{cm} \times 5 = 90 , \text{cm} \]
Therefore, the radius of the original circle is 90 cm.