[tex]\rm A~quarter~of~an~hour~\to~un~sfert~de~ora~\to~60:4=15~min~(Jim). \\ \\ Half~of~a~quarter~\to~jumatate~de~sfert~\to~60:4:2=7,5~min~(Jack). \\ \\ Half~an~hour~\to~jumatate~de~ora~\to~60:2=30~min~(Helen). \\ \\ Ten~minutes~\to~10~min~(Ana). \\ \\ Deci~raspunsul~este~\boxed{\rm a}.[/tex]