if two sets A and B are having 99 elements in common, then the number of elements common in each of the sets A x B and B x A are

Dear
A and b having 99 elements in common . 
Then number of elements common to each of the sets a × b and b × a = 99 × 99 
= (99)² = 9801 .

Actually, you can see choose 99 elements which are common to both a and b in 99 ways separately in both the cases .
And when you will multiple these you will get the same number of elements e.g., a × b = (99)² and b × a = (99)² 

So, the answer is 99² = 9801

Regards
 

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