To find out how many 18-inch pieces can be cut from a total of 1620 inches of ribbon, you divide the total length of ribbon by the length of each piece:
\[ \text{Number of pieces} = \frac{\text{Total length of ribbon}}{\text{Length of each piece}} = \frac{1620 \text{ inches}}{18 \text{ inches}} \]
Calculating this gives:
\[ \text{Number of pieces} = \frac{1620}{18} = 90 \]
So, there are 90 pieces of 18-inch ribbon that can be cut from 1620 inches of ribbon.