A student was asked to subtract (-12) from -47. He got -30. Is he correct? Justify.

A commuter train arrives punctually at a station every half hour. Each morning, a student leaves his house to the train station. Let x denote the amount of time, in minutes, that the student waits for the train from the time he reaches the train station. It is known that the pdf of X is f(x)={{:(1/30, 0 lt x lt 30), (0, "elsewhere"):} . Obtain interpret the expected value of the random variable X.

The teacher told the students that a frog originated from a fish. The students were shocked to hear this. They asked the teacher "How it could be possible". What did the teacher tell the boys to justify his statement.

A student was asked a question why are there summer and winter for us? He replied as since Earth is orbiting in an elliptical orbit. When the Earth is very far away from the Sun(aphelion) there will be winter, when the Earth is nearer to the Sun(perihelion) there will be winter. Is this answer correct? If not, what is the correct explanation for the occurrence of summer and winter?