$[$跟踪练习$ 2]$
$\frac {1}{7}×5 = \frac {5}{7}$
$1 - \frac {5}{7} = \frac {2}{7}$
$\frac {2}{7}÷6 = \frac {1}{21}$
$1 - \frac {1}{5} = \frac {4}{5}$
$\frac {1}{21}÷\frac {4}{5} = \frac {5}{84}$
$\frac {1}{7} - \frac {5}{84} = \frac {1}{12}$
$1÷\frac {1}{12} = 12(天)$
答:甲工程队单独完成这项工程要12天。