So the Rotten Apples Problem goes like this :
Question :
A box containing a dozen apples is opened and a few of the apples are found to be rotten. Which of the following may be the ratio of fresh apples to rotten apples ?
a). 4 : 7
b). 3: 1
c). 5 : 3
d).3 : 2
Solution :
This problem should take you five seconds. Not more 
Understand this : In Ratio problems like these, add all the ratios and see if it adds up to the total number or any of the factors of the total number. Here the total number is 12(because a dozen of apples)
See option a) … 4 + 7 = 11 …11 is not a factor of 12.
Similarly even c) and d) will fail. But option b) .. 3+1 = 4 which is a factor of 12 Hence b)3 : 1 is the right answer
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