Answer:
They buy [tex]\frac{7}{8}[/tex] of cheese.
Step-by-step explanation:
When adding two fractions with the same denominator, you simply add the two numerators (numbers on top):
[tex]\frac{3}{8} + \frac{4}{8} = \frac{7}{8}[/tex]
So they buy [tex]\frac{7}{8}[/tex] of cheese.