Translation:We drink a juice and you eat the rice.
Because it's like an idiom. Ieșim la (un suc, o cafea, o bere)? is a question you will often hear. It doesn't man you actually go out and drink only one juice. The problem here is the English translation. We drink juice should be the correct version.
If you translate it word by word it is 'a juice', even though it is more natural in English to lose the 'a'. But the lesson focuses on articles and understanding them.