Bunuel wrote:

If the remainder when positive integer y is divided by 9 is 3 and the remainder when y is divided by 6 is also 3, what is the smallest possible value of y?

A. 3

B. 12

C. 18

D. 21

E. 27

If the remainder when positive integer y is divided by 9 is 3

y= 9k+3 -->(1)

the remainder when y is divided by 6 is also 3

y= 6k+3 -->(2)

From (1) & (2) if k=0 then it is 3

Hence A

Also you can use Ans options, which gives 3 & 21

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Resource: GMATPrep RCs With Solution