Answer:
mass ratio of A/B is 2:1
Step-by-step explanation:
Since the mass of box A = 10g
 mass of box B = 5g
 Mass of box C = mass of box A + mass of box 
A ratio compares two quantities. To find the ratio of the two boxes: 
 Ratio of A to B = 

 Ratio of A to B = 
 = 2
 = 2
The mass ratio is 2:1 i.e box A has twice the mass of B