Let s be the number of shorts and t be the number of T-shirts. s shorts cost $12s and t T-shirts cost $5t, then you have to find min and max value for the function f(s,t)=12s+5t.
The shaded domain (see image) is defined from the system of unequalities. The green lines are the graphs of function f(x,y) and it intersects domain in first point (0,5) (the minimum point) and in last point (20,0) (the maximum point). So,
