Philip celebrates his 10th birthday. He found out that his year is the product of the date : 29/3/1987; 29x3=87. He also found out that his two little sisters have also the same feature when they celebrate their 10th birthday too. The DATE of the younger sister's 10th birthday is earlier by the older sister's by 80 days. If known that the younger sister's birthday is on 24 April 1996, how many years apart were the two sisters born?