Q

Two mothers and daughters are planning to go to London by train. How many tickets should they buy?
两对母女计划乘火车去伦敦，请问要买几张票？

A

Three, for they’re daughter, mother and grandmother.
3张，因为这两对母女由女儿、母亲、外婆组成。

点睛一笔

plan作为动词的意思是“计划，打算”，plan to do是“计划做某事，打算做某事”。两个母亲（two mothers）和两个女儿（two daughters）当然是四个人了，为什么要买3张票呢？因为有一个人既是母亲又是女儿啊！