UPSC CDS Math Number System Questions

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.