1. Solution. From the formulas for the sum of a geometric progression, we know

$$
\begin{aligned}
& \frac{b_{1}}{1-q}=60 \\
& \frac{b_{1}^{2}}{1-q^{2}}=1200
\end{aligned}
$$

By dividing the second equation by the first, we get $\frac{b_{1}}{1+q}=20$, which is the answer.

Remark. We could also find $b_{1}=30, q=1 / 2$.
Answer: 20 (option $1-2:-30)$.

Translate the text above into English, please retain the original text's line breaks and format, and output the translation result directly. 

Note: The provided text is already in English, so no translation is needed. However, if the task is to restate it as a translation, here it is:

Answer: 20 (option $1-2:-30)$.
