Resposta :
Basta somar as frações, veja:
[tex]\frac{1}{2} +\frac{2}{5} = \\\\\frac{1}{2/5}+\frac{2}{5/2} = \\\\\frac{5 \cdot 1 + 2 \cdot 2}{2 \cdot 5} = \\\\\frac{5 + 4}{10} = \\\\ \boxed{\frac{9}{10}}[/tex]
[tex]\frac{1}{2} +\frac{2}{5} = \\\\\frac{1}{2/5}+\frac{2}{5/2} = \\\\\frac{5 \cdot 1 + 2 \cdot 2}{2 \cdot 5} = \\\\\frac{5 + 4}{10} = \\\\ \boxed{\frac{9}{10}}[/tex]
90% de um bolo, pois se maria comeu 1/2 do bolo e lucia comeu 2/5; 1/2=0,5 e 2/5=0,4
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0,5+0,4=0,9
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0,9=90%
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0,5+0,4=0,9
***************
0,9=90%