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

<result>20</result>