1.都可译为：轮流做某事,
take turns to do sth 指做以前没做过的事情
take turns doing sth指接着做未完成的事情
1.our parents was out.my sister and I took turns to look after the baby.我的父母不在家,我和我姐轮流着照顾这个婴儿.
2.I have not finished the work yet,let us take turn doing the rest.
我仍然没有完成这项工作,我们轮流着把剩下的完成吧.
2.最高级和变间接引语请参考如下地址,介绍的非常详细.
3.设容积为v 两次共倒出纯药液6+6*（v-6）/v=12-36/v 共加水v-（12-36/v）所以【12-36/v】/【v-（12-36/v）】=16/9最后解得v=15或15/4 因为v>6所以v=15