I'll play a trick on you

Please look the picture carefully. Then I'll give you two integers and your task is output the third one.
Please never doubt the picture.

The first line is a number T(1<=T<=30), represents the number of case. The next T blocks follow each indicates a case.
Each case contains two integers A,B (1<=B<=A<=10¹⁰⁰)

For each case, output the number of case and the third integer.(as shown in the sample output)

3
99 72
45 27
39 18

Case 1: 27
Case 2: 18
Case 3: 21

If you have any idea to work out the ? and explain why but couldn't get Accepted , please email me (notonlysuccess@gmail.com), the first person will get 100RMB from me.

NotOnlySuccess

2011 Alibaba Programming Contest