8th grade pre-algebra ? ! i think the answer is c, but i'm just verifying.

Given this information, which expression must represent the value of b?

Acyclist bike x distance at 10 miles per hour .and returns over the same path at 8 miles per hour. what is the cyclist average rate for the round trip in miles per hour ?

1. if events a and b are non-overlapping events, how do you find the probability that one or the other occurs? 2. what does it mean if p(a or b) equals 1?