Calculate the positive number which when added by 15 is equal to 100 times the reciprocal of the number.

Correct option is C

Given:

Find the positive number x such that:

​$$x + 15 = 100 \times \left( \frac{1}{x} \right)$$​​

Solution: 

Now, 

​$$x + 15 = \frac{100}{x}$$​​

x(x + 15) = 100

​$$x^2 + 15x = 100$$​​

​$$x^2 + 15x - 100 = 0$$​​

$$x^2+20x-5x-100 = 0 \\ \ \\ x(x+20) -5(x+20) = 0 \\ \ \\ (x-5)(x+20) = 0 \\ \ \\ x -5 = 0 \ \ \ \ or \ \ \ \ x +20 = 0 \\ \ \\ x = 5 \ \ \ \ or \ \ \ \ x = -20$$​​​

So, positive number is 5.