A book has 3 chapters. The first chapter is 35 pages long the second chapter is 18 pages long and the third chapter is 3 pages long. How many more pages does the second chapter have than the third chapter?
Let's denote the number of pages in the first chapter by a, the number of pages in the second chapter by b, and the number of pages in the third chapter by c. 

Then we have the following system of equations: 

a=35

b=18

c=3

The difference between the number of pages in the second chapter and the number of pages in the third chapter is 

b-c=15. 

Therefore the answer (numeric) is: 

15