Answer:
You can't add apples to oranges. (You can't add grams and mililiters and get grams.)
Explanation:
Yvonne is mistaken because she does not account for the units. Look at the question again. She is adding grams (mass) with milliliters (volume). She simply cannot do this because they are not the same units.
"[In] the real world, many numbers have units associated with them. When we are dealing with two numbers that are expressed in units, if the units are the same, we can add or subtract normally. However, if the units are different, then we can add or subtract only if the units can be converted into the same units." - GMAT FREE
If we really wanted to add the two together, we'd use the property of density (the mass per unit volume of a substance) to do so. For example, if I wanted to add grams TO grams instead of grams TO mililiters, I'd have to convert the mililiters TO grams. We do this by using the density formula:
Then, after we have the same units, we can perform computation as usual.