Tim enters a competition and has to try and guess the number of red jelly beans in a jar.  The jar has a square base and is 6 inches by 6 inches and 15 inches tall.  He knows that a jelly bean is .15 cubic inches.  He also knows that they get about 80% efficiency in packing.  He estimates 30% of the jelly beans are red.  The true number of red jelly beans in the container is 900.  How far off was his guess?
The volume of the container is 6*6*15=
<gadget id="calculator">6*6*15</gadget>
<output>540</output>
540 cubic inches.
That means it can fit 540/.15=
<gadget id="calculator">540/.15</gadget>
<output>3_600</output>
3600 jelly beans.
With 80% packing efficiency, you can fit 3600*.8=
<gadget id="calculator">3600*.8</gadget>
<output>2_880</output>
2880 jelly beans in there.
So that means he guesses 2880*.3=
<gadget id="calculator">2880*.3</gadget>
<output>864</output>
864 are red.
So his guess is off by 900-864=
<gadget id="calculator">900-864</gadget>
<output>36</output>
36.

<result>36</result>