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Q-4: How many three digit numbers of distinct digits can be formed by using digits 1, 2, 3, 4, 5, 6 and 7 such that the numbers are divisible by 11?
A) 14
B) 16
C) 18
D) 20
E) 22
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CONCEPT: A number is divisible by 11 when Difference of Sums of even place digits and odd place digits is either zero or a multiple of 11.
i.e. a 3-Digit Number abc will be divisible by 11 if (a+c) - b = 0 or multiple of 11@b=1, a+c should be 1 or 12 i.e. (a,c) can be (5, 7), (7, 5) --- 2 cases
@b=2, a+c should be 2 or 13 i.e. (a,c) can be (6, 7), (7, 6)
---- 2 cases@b=3, a+c should be 3 or 14 i.e. (a,c) can be (1, 2), (2, 1)
---- 2 cases@b=4, a+c should be 4 or 15 i.e. (a,c) can be (1, 3), (3, 1)
---- 2 cases@b=5, a+c should be 5 or 16 i.e. (a,c) can be (1, 4), (2, 3), (3, 2), (4, 1)
---- 4 cases@b=6, a+c should be 6 or 17 i.e. (a,c) can be (1, 5), (2, 4), (4, 2), (5, 1)
---- 4 cases@b=7, a+c should be 7 or 18 i.e. (a,c) can be (1, 6), (2, 5), (3, 4), (4, 3), (5, 2), (6, 1)
---- 6 casesTotal Cases = 2+2+2+2+4+4+6 = 22 casesAnswer: option E