a)

$\frac{1}{3}+0,25=\frac{1}{3}+\frac{1}{4}=\frac{4}{12}+\frac{3}{12}=\frac{7}{12}$

b)

$0,36-\frac{2}{9}=\frac{36}{100}-\frac{2}{9}=\frac{9}{25}-\frac{2}{9}=\frac{81}{225}-\frac{50}{225}=\frac{31}{225}$