RDA wants you to take in 60 mg of Vitamin C A Day. A Grapefruit has 70 mg per 100 g. How many 6oz GF's would you need to equal this?

It's all a matter of conversion and proportions.

Convert 6oz into grams, or 85.7grams into oz.

The amount of grams you have is the weight of one grapefruit. By proportions, you know that you need 85.7g of grapefruit. Dividing the amount of grams in 6oz by 85.7g, you'll have the amount of grapefruit in a day.

If you choose to convert 85.7 grams into oz. Divide this number by 6 oz to obtain the answer.