Complex sentence: As a teenager she competed in the butterfly and medley stroke swimming, becoming the national butterfly and medley record holder (1958--59) and winning the silver medal in the 110 yards butterfly at the 1958 British Empire and Commonwealth Games.
Split into simpler sentences: As a teenager she competed in the butterfly and medley swimming events, becoming a national record holder in both events during 1958--59. She went on to win the silver medal in the 110 yards butterfly at the 1958 British Empire and Commonwealth Games.