From statement I: A two digit number, greater than 9 and multiple of 51 should be 51 itself. Because, 2 x 51 = 102 (3 digit number). Therefore, I alone sufficient to answer. From statement II: A two digit number, greater than 9 and sum of the digit is 6. It can be 15, 24, 33, 42, 51. So we cannot determine the required answer from the statement II alone. Thus, I alone give the answer while II alone not sufficient to answer.