UPSC CDS Math Number System Questions

The unit digit of 7n is 7 or 9 or 3 or 1 when the remainder obtainedon dividing n by 4 is 1 or 2 or 3 or 4, respectively
139=4×34+3
The remainder obtained on dividing 139 by 4 is 3.
So, the unit digit of 7139 is 3.