a WHOLE is always simplified to "1", namely 100/100 = 1, 57/57 = 1 and so on. Well, in this case we have a total of 21 students, so 21/21 is all of them, their "whole", but we also know that 6 of them read about animals and the rest about plants, so
[tex]\stackrel{whole}{\cfrac{21}{21}}\qquad \qquad \cfrac{\stackrel{animals}{6}~~ + ~~\stackrel{plants}{15}}{21}\implies \cfrac{6}{21}~~ + ~~\stackrel{plants}{\cfrac{15}{21}} \\\\[-0.35em] ~\dotfill\\\\ \cfrac{15}{21}\implies \cfrac{3\cdot 5}{3\cdot 7}\implies \cfrac{3}{3}\cdot \cfrac{5}{7}\implies 1\cdot \cfrac{5}{7}\implies \boxed{\cfrac{5}{7}}[/tex]
