Book Maths Puzzle Numerical Ability Permutation and Combination

in a class two students aged 16yrs and 18 yrs are replaced by two students whose age is 19yrs and 21 yrs.if the avearge age of the cls os increased by 3 months.?How many students are there in the class?

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Book Other Question

between June 1990 and February 1993,how many months have 31 days? Find total no ways in which a black square and a white square can be selected from a chess board such that both the square lies either in same row or in same column?
a) 256
b) 512
c) 128
d) 64