20 students
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Explanation:
Make a table that has "Year 10", "Year 11", and "Total" as the three rows.
Then you'll have the columns "French", "Spanish", "German", "Total".
In the bottom right corner is the value 100. This is the grand total of everyone surveyed.
We're told that 54 people are in Year 10, which leaves 100-54 = 46 people in Year 11.
Along the "total" column on the far right side, we'll have 54, 46, and 100 in that order from top to bottom.
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The info that "20 of the 29 people who study Spanish in Year 11" tells we have 20 people in year 11 who study Spanish, out of 29 people who study Spanish. That leaves 29-20 = 9 people in Year 10 who study Spanish. In the "Spanish" column, we have the values 9,20,29 in that exact order from top to bottom.
Next, we'll write 31 at the bottom of the "German" column to indicate 31 people study German.
The last bit of info is that "15 Year 11 students study French". So we'll write 15 in the "year 11" row and "french" column.
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After all that info is sorted out and entered into the table, you should end up with what is shown in figure 1 (see attached image below). Ignore figure 2 for now as we'll work our way to getting there eventually.
The variables a,b,c,d are placeholders for positive whole numbers.
There are probably a few approaches to take, but possibly the quickest is to notice that in the second row, we can say
15+20+c = 46
c+35 = 46
c = 46-35
c = 11
We have 11 students who study German in Year 11.
Once you determine c, it helps us find b. Focus on the German column to see that
b+11 = 31
b = 31-11
b = 20
There are 20 students in Year 10 who study German. You could stop here if you want.
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Just two more variables to go.
Along the top row, we see that
a+9+20 = 54
a+29 = 54
a = 54-29
a = 25
and along the bottom row we see that
d+29+31 = 100
d+60 = 100
d = 100-60
d = 40
Or you could note along this first column to say
a+15 = d
d = a+15
d = 25+15
d = 40
So there are a few alternative paths to take.
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All of the sections mentioned so far only involve filling out the table that is shown below in figure 2.
To answer the question your teacher wants to ask, which is how many Year 10 students study German, well that's simply 20 students. Refer to the conclusion of the previous two sections earlier.
By this point, we just simply copy the value from the completed table. This value being of course in the "Year 10" row and "German" column.
