To find out how many shoppers were buying food for a week or more, you can use the percentage given (40%) and apply it to the total number of shoppers surveyed (50).
Here's the calculation:
\[ \text{Number of shoppers buying food for a week or more} = \text{Total shoppers} \times \text{Percentage buying food for a week or more} \]
\[ \text{Number of shoppers} = 50 \times 0.40 = 20 \]
Therefore, 20 shoppers were buying food for a week or more.