Ta có: $v=\dfrac{s}{t}=\dfrac{L}{t}$
$L=6a+l$
$\Rightarrow v=\dfrac{L}{\frac{a}{v}+\frac{a}{2v}+\frac{a}{3v}+\frac{a}{4v}+\frac{a}{5v}+\frac{a}{6v}+\frac{l}{7v}}$
$=\dfrac{420v.L}{\frac{420a+210a+140a+105a+84a+70a+60l}{420v}}$
$=\dfrac{420v.L}{1029a+60l}$
$=\dfrac{420v.L}{1029a+60(L-6a)}$
$=\dfrac{420v.L}{1029a-360a+60L}$
$=\dfrac{420v.L}{669a+60L}$
$=\dfrac{140v.L}{223a+20L}$