Since there are 7 months from January to July, then Roe's savings for those months sum up to $10 x 7 = $<<7*10=70>>70.
Since there are 4 months from August to November, then her savings for those months sum up to $15 x 4 = $<<4*15=60>>60.
Her total savings from January to November is $70 + $60 = $<<70+60=130>>130.
Therefore, she needs to save $150 - $130 = $<<150-130=20>>20 in December.
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