Answer:
39 miles
Explanation:
The given distances are at right angles to each other, so form the legs of a right triangle whose hypotenuse is the straight-line distance of interest. The Pythagorean theorem can be used to find the length of the hypotenuse.
__
Your familiarity with Pythagorean triples lets you recognize the ratio of the distances, 15/36 = 5/12, means the triangle of interest is a multiple of the (5, 12, 13) Pythagorean triple. The multiplier is 3, so ...
the direct distance to her aunt's house is 39 miles.
__
Additional comment
The Pythagorean theorem tells you the square of the hypotenuse is the sum of the squares of the other two sides:
c² = a² +b²
c = √(15² +36²) = √1521 = 39 . . . . hypotenuse for legs 15 and 36
__
Some Pythagorean triples you will run across in algebra, geometry, and trig problems are ...
(3, 4, 5), (5, 12, 13), (7, 24, 25), (8, 15, 17), (9, 40, 41)
As here, you will also see multiples of these.