a) $\frac{75}{100}\cdot 28=\frac{3}{4}\cdot 28=3\cdot 7=21$  

$28+21=49$

b) $\frac{0.5}{100}\cdot 4=\frac{2}{100}=0.02$

$4+0.02=4.02$

c) $\frac{110}{100}\cdot 52=\frac{11}{10}\cdot 52=1\frac{1}{10}\cdot 52=52+\frac{1}{10}\cdot 52=52+5,2=57,2$  

$52+57.2=109.2$

d) $\frac{24}{100}\cdot 45=\frac{12}{50}\cdot 45=\frac{12}{10}\cdot 9=\frac{108}{10}=10,8$  

$45-10.8=34,2$

e) $\frac{0.1}{100}\cdot 250=\frac{25}{100}=0.25$

$250-0.25=249.75$

f) $\frac{2.5}{100}\cdot 32=\frac{25}{1000}\cdot 32=$   $\frac{1}{40}\cdot 32=\frac{32}{40}=\frac{8}{10}=0,8$  

$32-0.8=31.2$