ITMC 2019 Grade 6 National Preliminary set01 Question 19

A pattern of fractions is $\frac{11}{13},\frac{13}{24},\frac{15}{56},\frac{17}{72},\ldots,\frac ab$, where $\frac ab$ is the 15th term. It can be written as $\frac1x+\frac1y$. Find $(a+b)-(x+y)$.