Quantitative Aptitude :: Permutation and Combination - Discussion

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Explanation:
When 2 is at unit place=9*10*10*1
When 2 is at tenth place=9*10*1*10
When 2 is at hundred place=9*1*10*10
When 2 is at thousand place = 1*10*10*10
So Total no. = 900 + 900 + 900 + 1000 = 3700