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There are twelve rooms in a building and each room has two windows and one light. Last evening, eighteen windows were lit. In how many rooms was the light off? (2016 Math Kangaroo Problem, Level 1-2, Question \#14) 
Overseas Competition->Knowledge Point->Word Problem Modules->Unitary Method Problems->Finding the Total Value using Unitary Method
$18\div2=9$, $12-9=3$. 