Hello!
Martin buys more fruit, because 6/3 is greater than 4/3. You can check which fraction is greater by converting it into a decimal, as decimals are sometimes easier to compare.
4 / 3 = 1.3 (Repeating)
6 / 3 = 2
As you can see, 2 is greater than 1.3 Repeating, so 6/3 pounds are greater than 4/3 pounds.