Tomato juice has a pH of 4. The concentration of H+ in tomoato juice is how many times greater than seawater

Do you know the definition of pH? I'm sure it's in your text, but the key point is that a change of one point lower in pH means 10 times higher concentration of hydrogen ions. The text should also provide the pH of sea water, which may range anywhere from 7.5 to 8.4, so the answer will depend on the specific value you use.

You need another pH to figure this out. pH is logarithmic so it is not a direct proportional ratio.

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Originally Posted by ma0641

You need another pH to figure this out.

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Yes, as already noted he needs the pH of sea water in order to figure out how much greater the concentration of H+ is in tomata juice.