While a "given" is taken as true without proof, and a "problem" sometimes involves an assertion that can be proven, the answer choices involving these terms are not correct here.
The dictionary definitions of "postulate" and "theorem" apply. It is useful to learn the meaning of these terms so you can understand problems and descriptions that use them.
First you find the median, the middle term (if there are an even number of terms it is the average of the middle two terms.) Since you have 21 terms, odd, you just use the middle term, in this case 8. The first quartile works the same way, but just on the left half between the start and the median...so the first quartile is 6.
