Answer:
Step-by-step explanation:
Question (1). An alloy contains zinc and copper in the ratio of 7 : 9.
If the weight of an alloy = x kgs
Then weight of copper = 
= 
And the weight of zinc = 
= 
If the weight of zinc = 31.5 kg
31.5 = 
x = 
x = 72 kgs
Therefore, weight of copper = 
= 40.5 kgs
2). i). 2 : 3 = 
4 : 5 = 
Now we will equalize the denominators of each fraction to compare the ratios.
= 

Since, 
Therefore, 4 : 5 > 2 : 3
ii). 11 : 19 = 
19 : 21 = 
By equalizing denominators of the given fractions,

And 
Since, 
Therefore, 19 : 21 > 11 : 19
iii). 


= 
Now we equalize the denominators of the fractions,

And 
Since 
Therefore,
will be the answer.
IV). 



Similarly, 
By equalizing the denominators,

Similarly, 
Since 
Therefore, 
V). If a : b = 6 : 5



And b : c = 10 : 9

Since a : b = 12 : 10
And b : c = 10 : 9
Since b = 10 is common in both the ratios,
Therefore, combined form of the ratios will be,
a : b : c = 12 : 10 : 9