To determine how many napkins should be ordered for 15 guests based on the initial order, we can set up a proportion. The planner originally ordered 400 napkins for 8 guests.
First, find the number of napkins per guest:
\[ \text{Napkins per guest} = \frac{400 \text{ napkins}}{8 \text{ guests}} = 50 \text{ napkins per guest} \]
Now, to find out how many napkins are needed for 15 guests:
\[ \text{Napkins for 15 guests} = 15 \text{ guests} \times 50 \text{ napkins per guest} = 750 \text{ napkins} \]
Therefore, the planner should order 750 napkins for 15 guests.