UPSC Civil Services 2015 Prelims General Studies Paper II

If mathematics I is not opted, then two subjects out of four subjects have to be opted for.
∴ Number of ways in which two subjects can be opted for

4×3

2

=6
It mathematics II is opted, then it can be offered only if mathematics I is also opted for Number of ways in which two subjects can be opted for
= 6 +1 = 7.