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(1) The tens digit of n is a factor of the units digit of n >
lets consider the number is 10X +y ; it says Y=kX.. so the number will becose 10X+KX = X (10 +k) which will be a composite number unless X=1 ; now X can not be 1 as 10X+Y> 20
also if you read this option properly the, number shoul be 10X+Y < 49
(1) The tens digit of n is a factor of the units digit of n.
If 9, factors are 9,3,1 = 19,39,99 If 8, factors are 8,4,2,1 = 18,28,48,88 If 7, factors are 7,1 = 17,77 If 6, factors are 6,3,2,1 = 16,26,36,66 If 5, factors are 5,1 = 15,55 If 4, factors are 4,2,1 = 14,24,44 If 3, factors are 3,1 = 13,33 If 2, factors are 2,1 = 12,22 If 1, factors are 1 = 11
None of the # above 20 are prime, so it is composite.
(1) The tens digit of n is a factor of the units digit of n. ex: any number greater than 20, and tens digit is a factor of the units digit. 24, 26, 28, 36,33....none r prime...so they have to be composite
So, A is sufficient
(2) The tens digit of n is 2. ex: 21,22,23,24,25....could be prime or composite
Good question indeed ... I decided the answer was C before reading through the solutions below... I guess I need to be more careful with DS .... _________________
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