<p>A horse rider covers half the distance with&nbsp; 5m/s speed. The remaining part of the distance was travelled with speed 10 m/s&nbsp; &nbsp;for half the time and with speed 15 m/s&nbsp; for other half of the time. The mean speed of the rider averaged over the whole time of motion is&nbsp;<span class="equation">\(\frac{x}{7}\)</span>&nbsp;m/s. The value of x is __</p>