Stacey,

Would it be fair to say that the main idea of the RC passage is typically located in the 1st sentence of the passage, and then usually hit again in the last sentence of the passage?

not always. and especially not in longer r.c. passages, whose thesis statements are often found near the end of the first paragraph.

on shorter passages there is a high probability that the first sentence will have something to do with the thesis of the passage, but even that is by no means certain. many passages - even shorter ones - have an introductory sentence or two whose sole purpose is to set the stage for the topic sentence that follows.

also, it is extremely rare for the main idea of a passage to be 'hit again' anywhere. these passages aren't that long, so they usually cite their thesis exactly once, and then expand upon that thesis. it's common for the last sentence to change the direction of the thesis in some way - whether by qualifying it, or presenting some sort of counterpoint, or presenting some alternate angle on it, or whatever - but the last sentence will very rarely recapitulate the thesis itself. in other words: these passages rarely have anything that a high-school english teacher would call a 'concluding paragraph'.