Four small squares on a chess board are selected at random. the probability that they form a square of the size 2 X 2 is ab, where HCF of a and b is 1, then a+b=?
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Solution
Total number of ways of selecting 4 squares is 64. Number of ways of selecting square of size 2×2 =7×7=49 ∴ Required probability =4964C4=790768 Therefore a+b=7+90768=90775