Answer: The mall is 12 miles from his home to the library.
Explanation:
Since we have given that
Number of miles is from his home from the mall = 15 miles
The school is
of the way from his home to the mall.
The library is
of the way from your school to the mall.
According to question, it becomes,

So, the number of miles from his home to the library is

Hence, the mall is 12 miles from his home to the library.