$\frac{20}{2x7}$ + $\frac{20}{7x12}$ + $\frac{20}{12x17}$ + $\frac{20}{17x22}$ + $\frac{20}{22x27}$ + $\frac{20}{27x32}$
= 4 x ($\frac{5}{2x7}$ + $\frac{5}{7x12}$ + $\frac{5}{12x17}$ + $\frac{5}{17x22}$ + $\frac{5}{22x27}$ + $\frac{5}{27x32}$
= 4 x ($\frac{1}{2}$ - $\frac{1}{7}$ + $\frac{1}{7}$ - $\frac{1}{12}$ + $\frac{1}{12}$ - $\frac{1}{17}$ + $\frac{1}{17}$ - $\frac{1}{22}$ + $\frac{1}{22}$ - $\frac{1}{27}$ + $\frac{1}{27}$ - $\frac{1}{32}$)
= 4 x ($\frac{1}{2}$ - $\frac{1}{32}$)
= 4 x $\frac{15}{32}$
= $\frac{15}{8}$
Vậy đáp án đúng là: A
