To find out how much Sam spends on groceries each week, you need to calculate 15% of her weekly earnings.
First, calculate 15% of $400:
\[ 15% = \frac{15}{100} = 0.15 \]
Now, multiply her weekly earnings by 0.15:
\[ \text{Grocery spending} = 400 \times 0.15 = 60 \]
So, Sam spends $60 on groceries every week.