Concept:Apply the same mathematical operation to the whole first number to get the second number.Explanation:The pattern is: first number ×9+18= second number.For X:243, we have X×9+18=243.So, X×9=243−18=225.Thus, X=225÷9=25.For 17:Y, we have Y=17×9+18.So, Y=153+18=171.Answer:X=25, Y=171, which matches option C.