(+1/5)+(-8/15)=

$$\frac{1}{5}+\frac{-8}{15} =\frac{1}{5}-\frac{8}{15} =\frac{3}{15}-\frac{8}{15} =-\frac{5}{15} =-\frac{1}{3}$$

+ * - = -

$$\frac{1*3}{5*3}=\frac{3}{15}$$ Erweitern

$$\frac{5:5}{15:5}=\frac{1}{3}$$ Kürzen

=3/15-8/15=-5/15=-1/3