Ответ:

решение не моё, в интернете нашла.

Verified answer

[tex]\displaystyle\bf\\b_{1} =6\\\\q=\frac{1}{10} =0,1\\\\b_{n} =0,006\\\\n=?\\\\\\b_{n} =b_{1} \cdot q^{n-1} \\\\0,006=6\cdot 0,1^{n-1} \\\\0,1^{n-1} =0,006:6\\\\0,1^{n-1} =0,001\\\\0,1^{n-1} =0,1^{3} \\\\n-1=3\\\\\boxed{n=4}[/tex]