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Note - I am not a math teacher; I am seeking an answer from a math teacher.

While helping my son with his 5th grade homework today we had one answer wrong. I'm not sure I agree with the book and wanted to see if there's a rule on this. The assignment is to write the expression for the phrase.

The problem is 2/3 of 30 minus 11. You can read that two ways: "Two thirds of [pause] thirty minus eleven" "Two thirds of thirty [pause] minus eleven"

Our answer was (2/3 * 30) - 11 and we are aware the parenthesis do not matter here. This also fits with the fact that 30 is divisible by 3. The book's answer is 2/3 * (30 - 11).

Is there a specific reason for this? Misprint?

I'll add that the book lists the answer to 1/4 times 8 increased by 11 as being 1/4 * 8 + 11 which we have correct. Seems quite similar.

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    $\begingroup$ Keep in mind order of operations. But I would interpret that as $2/3 \times 30 -11$. $\endgroup$
    – Chris C
    Mar 16 '15 at 19:08
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    $\begingroup$ Since "of" is read in this context as signifying multiplication, I would agree that you have $2/3 \times 30 - 11$, which, using the order of operations, would be your answer (and not the book's!). $\endgroup$
    – Benjamin Dickman
    Mar 16 '15 at 20:10
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    $\begingroup$ I might add that as currently written, it is better suited for math.SE. But I think it is a good question if we adjust to how to word such problems in an education context. $\endgroup$
    – Chris C
    Mar 16 '15 at 20:39
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it sounds like the textbook has some poorly written and vague questions. there is no reason why either of the answers would be "better" for the first example. It is simply dependent on how you interpret the words. You would be surprised how many textbooks actually include similar vagueness in some questions and it is concerning to say the least.... keep up the good work of teaching your son math! This might actually be a good opening to have a nice discussion with him about the ambiguities that are present and challenge him to come up with a similar question without any of the same ambiguities

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  • $\begingroup$ Thanks, will do. $\endgroup$
    – Paul
    Mar 16 '15 at 18:50

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