Two days apart

[dethwing]

Had a problem on a HW like this:

Solve the problem.

A course meets 5 times a week. One student misses class 2 days apart. A second student misses class 7 days apart. They were both absent on the second day of class. What is the next class day when both will be absent again?

My class and I apparently disagree on the meaning of "2 days apart". I said it meant 2 days later, so if you miss Monday, you miss Wednesday.
They [Or at least, some of them] argued that it meant there were 2 days in between. So if you Monday, you miss Thursday.

What do you folks say? Is the problem worded clearly? Are my students crazy?

[Trojan Horse]

It's ambiguous.

There are other ambiguities too:

1. You said the course meets 5 days a week. Which 5 days? (Don't laugh. Some colleges have courses on Saturdays.)

2. Even assuming that it is Monday through Friday, what happens on weekends? Let's pick an interpretation of what the first student does; let's say there are two days in between each missed class. What happens if that student misses class on Friday. When does he next miss class? Monday (with two days in between)? Wednesday (with two classes in between)? What if he misses class on Thursday; then when will his next miss be?

Too many ambiguities. Throw the question out.

[dethwing]

It was intended to mean class days. So if you miss Friday, you miss Tuesday. But I can see that being a problem.

[MatthewV]

Two days apart-- I would say it means missing Monday, being present for two days, and then missing another day.

If I was to see this on a test, I would clearly state my position and then solve with my assumptions. I would do this because I find the wording to be ambiguous.

[extropalopakettle]

[Zag]

In any case, "2 days apart" doesn't suggest repeating, like "every 2nd class" does. If you said, "the two times he missed class were only 2 days apart," then I don't think anyone would be confused. It's when you try to assume that it's a repeating thing that it starts to become awkward.

[Jedo the Jedi]

I thought it was a repeating thing, hence the nature of the question. "If these two students constantly miss at these intervals, at what point will their absences converge?"

I'm with extro, and I think the logic to answer the question doesn't require the clarity. Tell your whiny students they all fail.
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[Neo]

My first reading was your intended definition Dethy, but I did wonder how it would apply to a Friday.

The girlfriend interpreted it as some of your students did, and also asked about Friday.

FWIW, our backgrounds are similar. Physics with math minors, and some time working in education after. Had we been classmates and discussing the problem, it would have led us to asking you for clarification. Though it was fairly straightforward to solve, so the likelihood that we talked about it would be low, thus would have turned it in under differing interpretations.

We both feel it's linguistically ambiguous. It happens. I would see no problem with awarding full points to anybody who read it the other way and solved it correctly. They're still showing mastery of the issue at hand, which was how to approach and solve the problem. It would be wrong to punish them for the other interpretation.
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_{Ad Astra}

[dethwing]

The funny thing is, it's not my wording.

I took the class from someone else who has been using this wording for ages. I read it, knew instinctively what he was asking, and didn't give it a second thought. Until now.

[dethwing]

Then in the second class, someone asked about a different problem. It asked to find all values of d that make the following statement true:

9 | 78,32d

The student said he thought it was multiplication by d, instead of being a digit.

My immediate response was: despite the fact that there are two numbers to the right of the comma? It doesn't say digit, true, but who puts a comma after 2 digits?

[Death Mage]

"Two days apart" would mean, to me, Tuesday and Thursday, for example. Those are "two days apart".
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[Trojan Horse]

[Neo]

[MatthewV]

I tend to not use commas when handwriting numbers. I learned to put a small amount of space between groups of numbers.

For the 9 | 78,32d, I looked around to see where you defined d. And I still have no clue what the | means. So exactly what am I looking for?

[dethwing]

Vertical line means "divides". So 2 | 4.

[referee]

In other words, from numbers between 78320 and 78329 (both included), which ones are divisible by 9?
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[Lepton*]

SI/Metric/ISO demands a space to separate the thousands from the hundreds for numbers larger than 9999. ie: 10 000; 999 999; 4444. I've seen some Americans use a comma in numbers like 3333 (making it 3,333) but otherwise I think comma usage simply apes European spacing (10,000; 999,999).

For what it's worth, as a Canadian who has done some traveling, I'd have interpreted that comma as a decimal point, and thus misunderstood your question, at least on a first reading.

[Jedo the Jedi]

I don't see why it being a decimal would cause misunderstanding. That would just mean the number to be divided would be smaller.
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[extro...*]

[Chuck]

"He only drinks red wine" looks like it means no one else drinks red wine.

[Neo]

He only drinks red wine.

Only he (and nobody else) drinks red wine.
The only action he performs with red wine is drink it.
The only alcoholic drink he consumes is red wine.
The only red thing he drinks is wine.
The only liquid he ingests is red wine.
The only thing he ingests (period) is red wine.
He does nothing else at all except drink red wine.
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_{Ad Astra}

[MatthewV]

And on another side note: when writing decimal numbers less than one, always add the leading zero. Otherwise, people will commonly not see the decimal point. This is particularly true with single digit decimals.
A few months ago I added 10 times too much cobalt carbonate to a glaze recipe because it was written as .5. It was a very rich dark blue that everyone loved (and cost ~$50 more)

Written language should be about clarity first and elegance second.

[Jedo the Jedi]

Sometimes clarity comes through elegance, just probably not in any of the fields you're thinking.
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[MatthewV]

A cylinder in the vertical arrangement, dances in side another
Diameters of 180 mm on one and 181.2 mm on the other
Each with a height of 300 mm, these things are really big!
So find the viscosity of the separating fluid, it doesn't require trig
When torque is 20 Nm is somehow applied or beckoned
So the smaller one rotates twice a second!