$\text{a)}40\%\cdot 210=\frac{4\sout{0}}{1\sout{00}}\cdot 21\sout{0}=84$  

$\text{b)}15\%\cdot 24=\frac{{\sout{15}}^3}{{\sout{100}}^{20}}\cdot 24=\frac{3\cdot {\sout{24}}^{12}}{{\sout{20}}^{10}}=\frac{36}{10}=3,6$  

$\text{c)}25\%\cdot 14=\frac{{\sout{25}}^1}{{\sout{100}}^4}\cdot 14=\frac{{\sout{14}}^7}{{\sout{4}}^2}=\frac{7}{2}=3,5$  

$\text{d)}18\%\cdot 240=\frac{18}{{\sout{100}}^5}\cdot {\sout{240}}^{12}=\frac{216}{5}=43\frac{1}{5}=43\frac{2}{10}=43,2$  

$\text{e)}72\%\cdot 25=\frac{72}{{\sout{100}}^4}\cdot {\sout{25}}^1=\frac{{\sout{72}}^{18}}{{\sout{4}}^1}=18$  

$\text{f)}75\%\cdot 38=\frac{{\sout{75}}^3}{{\sout{100}}^4}\cdot 38=\frac{3\cdot {\sout{38}}^{19}}{{\sout{4}}^2}=\frac{57}{2}=28\frac{1}{2}=28,5$  

$\text{g)}84\%\cdot 150=\frac{84}{{\sout{100}}^2}\cdot {\sout{150}}^3=\frac{{\sout{84}}^{42}\cdot 3}{{\sout{2}}^1}=126$  

$\text{h)}22\%\cdot 3500=\frac{22}{1\sout{00}}\cdot 35\sout{00}=770$  

$\text{i)}12\%\cdot 865=\frac{{\sout{12}}^3}{{\sout{100}}^{25}}\cdot 865=\frac{3\cdot {\sout{865}}^{173}}{{\sout{25}}^5}=\frac{519}{5}=103\frac{4}{5}=103\frac{8}{10}=103,8$