Mass box C is 10+5. (So C is 15)
But if C was 30, how many times could you put B (5) into it? 
30/5 = 6
You would need 6 boxes of B to make 30 grams of C.
        
                    
             
        
        
        
Answer:
0.6 grams of hydrogen are needed to react with 2.75 g of nitrogen.
Explanation:
When hydrogen and nitrogen react they form ammonia.
Chemical equation:
N₂ + 3H₂ → 2NH₃
Given mass of nitrogen = 2.75 g
Number of moles of nitrogen:
Number of moles = mass/ molar mass
Number of moles = 2.75 g / 28 g/mol
Number of moles = 0.098 mol
Now we will compare the moles of nitrogen with hydrogen from balance chemical equation:
                    N₂             :          H₂
                     1               :           3
                    0.098       :        3×0.098 = 0.3 mol
Mass of hydrogen:
Mass = number of moles × molar mass
Mass = 0.3 mol × 2 g/mol
Mass = 0.6 g
 
        
             
        
        
        
Sodium mn2 and po4  makes sodium