If $\frac{1}{b - a} + \frac{1}{b - c} = \frac{1}{a} + \frac{1}{c}$,then $a, b, c$ are in

Given $a + d > b + c$ where $a, b, c, d$ are real numbers,then

An automobile driver travels from a plane to a hill station $120 \ km$ distant at an average speed of $30 \ km/hr$. He then makes the return trip at an average speed of $25 \ km/hr$. He covers another $120 \ km$ distance on the plane at an average speed of $50 \ km/hr$. His average speed over the entire distance of $360 \ km$ will be:

If $a, b, c$ are in $H.P.$,then the value of $\left( \frac{1}{b} + \frac{1}{c} - \frac{1}{a} \right) \left( \frac{1}{c} + \frac{1}{a} - \frac{1}{b} \right)$ is

Find the Harmonic Mean $(H.M.)$ of the sequence $\frac{1}{2}, \frac{1}{3}, \frac{1}{4}, \dots, \frac{1}{17}$.

If $a, b, c$ are in $G$.$P$. $(a > 1, b > 1, c > 1)$, then for any real number $x$ $(x > 0, x \neq 1)$, the terms $\log_{a} x, \log_{b} x, \log_{c} x$ are in: