This is a combination problem.
6 nCr 2

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Answer:
First of all, he have to find out what 0.5% of the students is.
Second of all, find out what 0.5 of 100 is. 100 divided by 0.5 is 200.
Third of all, divide 200 from 639. This equals 3.195
Since it is asking about how many, I would go with about 3. After all, you can't have have half of a person, so the decimals shouldn't be added.
Step-by-step explanation:
Answer to written question:
3x + 10x + 7x = 180
Combine like terms
20x = 180
Divide by 20
x = 9
Smallest angle 3x = 3*9 = 27 degrees
Answer to question in picture:
30/6x-2 = 36/5x + 13
Cross multiply
30(5x + 13) = 36(6x-2)
Distribute
150x + 390 = 216x - 72
Subtract 150x
390 = 66x - 72
Add 72
462 = 66x
Divide by 66
x = 7