Answer:
63 grams of protein in 9 ounces of tuna.
Explanation:
Equivalent ratios are a rule of three problem.
In this problem, we have that:
3 ounce serving of tuna provides 21 grams of protein. To find the equivalent ratio, how many grams of protein are there in one ounce serving of tuna?
3 ounces - 21 grams
1 ounce - x grams


The ratio is 7 grams of protein per ounce, so 7:1.
So in 9 ounces of tuna, there are 9*7 = 63 grams of protein.