Each of A, B, C, and D has Rs. 100. A pays Rs. 20 to B, who pays Rs. 10 to C, who gets Rs. 30 from D. In this context, which one of the following statements is not correct?

Consider the following matrix:

What is the missing number at ‘?’ in the matrix?

A selection is to be made for one post of Principal and two posts of Vice-Principal. Amongst the six candidates called for the interview, only two are eligible for the post of Principal while they all are eligible for the post of Vice-Principal. The number of possible combinations of selectees is:

Two cities, A and B, are 360 km apart. A car goes from A to B at a speed of $40$ km/hr and returns to A at $60$ km/hr. What is the car's average speed?

Two equal glasses of the same type are respectively $\frac{1}{3}$ and $\frac{1}{4}$ full of milk. They are then filled up with water and the contents are mixed in a pot. What is the ratio of milk to water in the pot?

The monthly incomes of Peter and Paul are in the ratio of $4:3$. Their expenses are in the ratio of $3:2$. If both save Rs. 6000 at the end of the month, their respective monthly incomes are: