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On her first day of work, Janabel sold one widget. On day two, she sold three widgets. On day three, she sold five widgets, and on each succeeding day, she sold two more widgets than she had sold on the previous day. How many widgets in total had sabing $20$ days? (2015 AMC 8 Problems, Question \#9) 
Overseas Competition->Knowledge Point->Calculation Modules->Sequences and Number Tables
We can easily find out she makes $2 \cdot 20-1=39$ widgets on Day $20$ . Then, we make the sum of $1,3,5, \ldots \ldots, 35,37,39$ by adding in this way: $(1+39)+(3+37)+(5+35)+\ldots+(19+21)$, which include $10$ pairs of $40$ . So the sum of $1,3,5, \ldots \ldots \ldots 39$ is $(40 \cdot 10)=(\text{D}) 400$ 