Lily\textquotesingle s family used $$21$$ tons of water in the first two months, and an average of $$31$$ tons of water on the remaining three tests. The average tons of water for the five months was~\uline{~~~~~~~~~~}~. 
Overseas Competition->Knowledge Point->Word Problem Modules->Average Problems 
Total tons of water: $$21\times 2+31\times 3=135$$  Average tons of water: $$135\div 5=27$$ 