Let Hong lives at a distance of d miles from mountains .
We have to use the formula of speed, distance and time, which is
[tex] speed= \frac{distance}{time} [/tex]
On droving to the mountain,
[tex] speed = \frac{d}{10}miles/hour [/tex]
On droving back to home,
[tex] speed =\frac{d}{7}miles/hout [/tex]
The difference between the two speeds = 18miles per hour, that is
[tex] \frac{d}{7}- \frac{d}{10}=18 [/tex]
[tex] 3d = 18*70 [/tex]
[tex] d = 420 miles [/tex]
