Telangana Police Constable Prelims 2018 Paper

Find a pair of words or numbers or letters or a combination of these which have a relation between them and a word of another pair, Identifying the relation between the words in the first pair, you have to pick up, among the four options, a word which has the same relation as in the first pair with the word given in the second pair.
20 : 21 ∷ ______ : N