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This is a standard problem applicable to mixtures as well as some problems on averages.

Code:

A                 :           B
a                             b
                        
                  c
    
(b~c)          :        (a~c)

What I've shown above is that if A and B are the items whose certain parameter is given as 'a' and 'b' respectively and same parameter of resultant mixture C is given to be 'c',
Then the ratio in which A and B have been mixed is (c~b) : (c~a)
['~' sign stands for difference not subtraction]