C3 Number of A.P’s with c.d’s 1,2,3,.........,9 are respectively 18, 16, 14,............, 2 Total no. of A.P’s = 2 + 4 + 6 +..........+ 18 = 2(1 + 2 + 3 +..................+ 9) = 90 No. of A.P’s with odd c.d. = 18 + 14+ 10 + 6 + 2 = 50 Sum = even => (3 nos. are even) or (1 even, 2 odd) No. of fav. Case -