$A$ $500 \ kg$ car takes a round turn of radius $50 \ m$ with a velocity of $36 \ km/h$. The centripetal force acting on the car is (in $N$)

Which one of the following elements shows the maximum number of different oxidation states in its compounds?

$x$ grams of calcium carbonate was completely burnt in air. The weight of the solid residue formed is $28 \text{ g}$. What is the value of $x$ (in grams)?

Starting from rest, the time taken by a body sliding down on a rough inclined plane at $45^{\circ}$ with the horizontal is twice the time taken to travel on a smooth plane of the same inclination and same distance. Then the coefficient of kinetic friction is:

Seven white balls and three black balls are randomly arranged in a row. The probability that no two black balls are placed adjacently is

Let $R_1$ be a relation defined by $R_1 = \{(a, b) | a \geq b, a, b \in \mathbb{R}\}$. Then $R_1$ is