Roe saved $10 per month from January to July and saved $15 per month from August to November. How much should she save in December so she will have a total savings of $150 in the year? Let's be accurate as possible.
Since there are 7 months from January to July, then Roe's savings for those months sum up to $10 x 7 = $70. Since there are 4 months from August to November, then her savings for those months sum up to $15 x 4 = $60. Her total savings from January to November is $70 + $60 = $130. Therefore, she needs to save $150 - $130 = $20 in December.
The answer: 20.