其它
Mary wanted to buy $12$ lemons. The lemons were sold either for $90$ cents each or at $3$ dollars for a bag of $4$ lemons. How much money would Mary save if she buys three bags of lemons instead of buying $12$ lemons separately? 
Overseas Competition->Knowledge Point->Calculation Modules
$0.9 \times 12 - 3 \times 3 = 1.8$ 