Question: which is not a common multiple of 4 and 9?
Answer: List the multiples of each number , then pick the smallest common one. multiples 4: {4,8,12,16,32,36,.....} multiples of 9: {9,18,27,36,45,.....} multiples of 3: {3,6,9,12,15,18,21,24,27,30,33,36,...}