We have the next formula
where d is the distance, r is the rate and t is the time.
For the trip going
d=r*7
For the trip returning
d=(r+27)*4
Then we solve
7r=4r+108
We solve for r
7r-4r=108
3r=108
r=36
Then for d
d=36*7
d=252 miles
ANSWER
she lives 252 miles far from the mountains