59. Given $0<a<1$, and it satisfies $\left[a+\frac{1}{30}\right]+\left[a+\frac{2}{30}\right]+\cdots+\left[a+\frac{29}{30}\right]=18$, then the value of $[10 a]$ is
$\qquad$ . ( $[x]$ represents the greatest integer not exceeding $x$)
answer: 6