If you would illustrate the problem, you will be able to formulate a triangle with the measurement of the hypotenuse (distance between the two shops) to be missing. Using the Pythagorean Theorem, the formula to compute for the hypotenuse is:
c = √a^2 + b^2
c = √(20)^2 + (13.5)^2
c = √400 + 182.25
c = √582.25
c = 24.13 km
Therefore, the distance between the two shops is approximately 24.13 km.