A horse rider covers half the distance with 5 m/s speed. The remaining part of the distance was travelled with speed 10 m / s for half the time and…

A horse rider covers half the distance with $\displaystyle 5$ m/s speed. The remaining part of the distance was travelled with speed $\displaystyle 10 \mathrm{~m} / \mathrm{s}$ for half the time and with speed $\displaystyle 15 \mathrm{~m} / \mathrm{s}$ for other half of the time. The mean speed of the rider averaged over the whole time of motion is $\displaystyle \frac{x}{7} \mathrm{~m} / \mathrm{s}$. The value of $\displaystyle x$ is $\displaystyle \_\_\_\_$.