Answer:
Mass of product B = 15g
Step-by-step explanation:
Reaction: 
 AB → A + B 
Mass of AB = 25g 
Mass of A = 10g 
Unknown: 
Mass of B = ? 
Solution: 
Based on the law of conservation of matter "matter is neither created nor destroyed but transformed from one form to another in a chemical reaction". 
 The amount of product = amount of reactant
 Mass of AB = Mass of A + Mass of B 
 Now; 
 Mass of Product B = Mass of AB - Mass of A
 Mass of product B = 25 - 10 = 15g 
Mass of product B = 15g