They both have the same denominator or bottom half of the fraction so you need to compare the numerators. 6 is more than 4 so Martin bought more fruit than Jack. Hope this helps.
Martin buys more fruit because he buys 6/3, and 6 is greater than 4, and Jack buys 4/3, so Martin bought more food than Jack. There's a difference of 2/3 between what Jack and Martin brought.
