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Let a = 10 and b = 30 be the two given sides
Call the third side c. We cannot find the value of c but we can find the range of what it could be.

It turns out that
b-a < c < b+a
where 'b' is larger than 'a'

So this means
b-a < c < b+a
30-10 < c < 30+10
20 < c < 40

So the third missing side is between 20 and 40. It cannot equal 20. It cannot equal 40. 

With this in mind, the answers are choice B) 21 and choice C) 35 since these values are between 20 and 40 (excluding the endpoints)

Note: I'm using the triangle inequality theorem

I’m sure you could google it. my profile picture might