Answer: There are 18 ounces of plum tomatoes he used.
Explanation:
Since we have given that
Number of pounds of grape, roma and plum = 4 pounds
As we know that
1 pound = 16 ounces
So, Number of ounces of grape, roma and plum =

Number of pounds of grape tomatoes = 2 pounds
Number of ounces of grape tomatoes would be

Number of roma tomatoes = 14 ounces
So, total number of ounces of grape and roma tomatoes is given by

so, Number of plum tomatoes would be

Hence, there are 18 ounces of plum tomatoes he used.