Hassem and his family cut an apple pie into 5 equal slices and ate 3.5 slices of it (35% of the apple pie). To find out how many slices of the blueberry pie they ate, we first need to determine the fraction of the apple pie they consumed.
They ate 3.5 out of 5 slices of the apple pie, which can be expressed as a fraction:
\[ \text{Fraction of apple pie eaten} = \frac{3.5}{5} = \frac{35}{50} = \frac{7}{10} \]
Now, we apply the same fraction of the blueberry pie, which has 10 slices:
\[ \text{Slices of blueberry pie eaten} = \frac{7}{10} \times 10 = 7 \]
So, they ate 7 slices of the blueberry pie. However, this is not one of the options listed. It appears there was an error with the options or statement of the problem.
If we strictly adhere to the original inquiry, 7 slices of the blueberry pie is the answer. If you're looking for one of the provided options based on other potential errors in the problem statement, we can't definitively match any of the provided numbers (6, 4, 5, 3) with the calculated correct answer of 7 slices.