Think of it this way: if the total track is a mile, and Jason runs 5/7 of it, he has run 5/7 of a mile. Then Sara runs 4/5 OF 5/7, with "of" meaning "times," so she runs 4/5 x 5/7, which gives you 20/35, which simplifies to 4/7. You can also think of Sara's distance as 80 percent of Jason's. If she runs "80 percent of the sevenths that Jason ran," that means she ran 4 out of his 5 sevenths.
