0,9(0,4-1 2/3м)-0,2(0,8-7 1/2м)=
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$0,9(0,4-1 frac{2}{3}m)-0,2(0,8-7 frac{1}{2}m)=[tex]frac{9}{10} ( frac{4}{10} -frac{5}{3}m)- frac{2}{10} ( frac{8}{10} -frac{15}{2}m)$ $= frac{9}{10} ( frac{2}{5} -frac{5}{3}m)- frac{1}{5} ( frac{4}{5} -frac{15}{2}m)=$ $frac{9}{10}* frac{2}{5} - frac{9}{10}*frac{5}{3}m- frac{1}{5}* frac{4}{5} +frac{1}{5}*frac{15}{2}m= frac{9*1}{5*5} - frac{3*1}{2*1} m- frac{1*4}{5*5}+ frac{1*3}{1*2}m=$ $frac{9}{25} - frac{3}{2} m- frac{4}{25}+ frac{3}{2}m= frac{9-4}{25}= frac{5}{25} = frac{1}{5}$