57. 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 $[10 a]$ $=\ldots$. ([x] represents the greatest integer not exceeding $x$, for example $[2.6]=2,[0.2]=0)$
Answer: 6