Respuesta :
if plus or minus the bottom of line must be the same number
but if devide or multiplication the bottom of line no need to be same number
but if devide or multiplication the bottom of line no need to be same number

[tex]\dfrac{1}{\frac{1}{2}}-\dfrac{3}{10}+\dfrac{1}{5}=1\cdot\dfrac{2}{1}-\dfrac{3}{10}+\dfrac{1\cdot2}{5\cdot2}=2-\dfrac{3}{10}+\dfrac{2}{10}=2-0.3+0.2=1.9[/tex]