A committee is reviewing a total of 20x black-and-white films and 4y color films for a festival. If the committee selects y/x% of the black-and-white films and all of the color films, what fraction of the selected films are in color? Choose the most appropriate option:
A) 26 / 21  B) 28 / 21  C) 20 / 21  D) 40 / 21  E) 60 / 21

Rationale: "say x = y = 10 . in this case we would have : 20 x = 200 black - and - white films ; 4 y = 40 color films . y / x % = 10 / 10 % = 1 % of the black - and - white films , so 2 black - and - white films and all 40 color films , thus total of 42 films were selected . color films thus compose 40 / 42 = 20 / 21 of the selected films . answer : c"
Answer: C