Jack buys 4/3 pounds of plums. martin buys 6/3 pounds of apples. who buys more fruit? explain your answer.

Since they already have the same denominator (bottom number) it is just a matter of which has a larger numerator (top number) In this case you are comparing 4/3 and 6/3
If you wanted to convert these into mixed numbers you could but that is just an extra step
4/3=1 1/3
6/3=2
Putting both of the numbers like that may be easier for you to see that 6/3 (or 2) is greater than 4/3 (or 1 1/3)
So Martin bought more fruit