If the average of a number, its 50% and its 25% is 210, the number is

The correct answer is (c).

Let $x$ be the number. We are given that the average of $x$, its 50% and its 25% is 210. This means that:

$$\frac{x + \frac{1}{2}x + \frac{1}{4}x}{3} = 210$$

Simplifying, we get:

$$\frac{7}{12}x = 210$$

Multiplying both sides by $\frac{12}{7}$, we get:

$$x = 360$$

Therefore, the number is 360.

Option (a) is incorrect because 280 is not a multiple of 7.

Option (b) is incorrect because 320 is not a multiple of 7.

Option (d) is incorrect because 400 is not a multiple of 7.