为鼓励居民节约用水,某市规定:每月用水量在20吨以内的,每吨水价1.8元;超过20吨的部分,每吨水价2.5元.小青家上个月缴水费43.5元.她家上个月用水____吨?
If you can't find your pen ,I would like to _____my pen to you.