Yes.
"My mathematics is good" is correct out of the choices given, but a better sentence would be "I am good at mathematics".
Yes, the sentence has a singular subject (one tragic effect), and a singular verb (massive flooding.)
You and your friend applied at....
It should be: I applied for duty yesterday.Had and on are not necessary.
The correct grammar would be "position applied for." This phrasing indicates that the position is the object of the action "applied."
i already have been appllied for my leave.
It's correct to say that a person is good AT mathematics.
she is good at mathematics
I have a B.A. in Mathematics would be correct.
Math's (as in, "math's a difficult subject for some people").
"How are you" is a correct sentence.
The correct spelling is 'mathematics'.