Katrina and Amanda are studying the same class. On Monday, Katrina solved 35 math problems and Amanda solved 21 math problems. On Wednesday Katrina solved 16 math problems and Amanda solved 20 math problems. On Thursday, Katrina solved 40 math problems and Amanda solved 24 math problems. On Saturday, Katrina solved 18 math problems and Amanda solved 24 math problems. On which day did Katrina and Amanda have the same ratio of problems as Monday?