Juanita used her affinity for numbers to help her design a computer program that relied on college-level math formulas, even though she was only in middle school. Based on the context clues in the sentence, what is the best meaning of “affinity”?

The best meaning of "affinity" in this passage would be "a natural liking for something." So Juanita natural likes and enjoys numbers.

the present continuous is a verb tense which is mainly used to indicates that an action is in progress at the moment of speaking.  

it is formed using: person + verb to be (am/is/are) +verb + - ing form  

example: i am studying at this moment .

in addition, the present continuous is also used in order to express things that will or will not happen in the near future.

example: i am having dinner with some friends tonight.

the speaker sometimes   uses it   to express that an action is repeated and it causes irritation on the part of the speaker

example: my sister is constantly complaining about everything.