UPSC 2016 CDS II Math Paper

S = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
a×b=12k+4,where a,b ∊ S
So, ab can take the values 16, 28, 40, 52, 64, 76 and 88.
The possible pairs (a, b) are (2, 8), (8, 2), (4, 7), (7, 4), (5, 8), (8, 5), (4, 10), (10, 4).
Thus, the possible number of pairs (a, b) are 8.