Nicole is 5 years old (Clue 3), David has a red lollipop (Clue 2), Maria is 6 years old (since she's the only other girl) (Clue 1), David is older than Maria (Clue 2) and Jonny is older than David (Clue 4), that makes David 7 and Jonny 8 -- therefore, since Nicole's lollipop isn't purple (Clue 3), she has the green lollipop!