Complex sentence: Tamsin Cook (born 1998) is an Australian competitive swimmer who won the gold medal in the 400 meter freestyle at the 2015 FINA World Junior Swimming Championships in Singapore in a new Championships record.
Split into simpler sentences: Tamsin Cook (born 1998) is an Australian competitive swimmer and the junior world champion in the 400-meter freestyle. She won the gold medal in the 400 meter freestyle at the 2015 FINA World Junior Swimming Championships in Singapore in a new Championships record.