The sixth grade office bought a pack of white paper and planned to use 20 sheets per day for 28 days. Due to paying attention to saving paper, I actually saved 4 sheets per day. How many days can I actually use? (Solve with ratio)
六年级办公室买进一包白纸，计划每天用20张，可以用28天．由于注意了节约用纸，实际每天节约了4张，实际可以用多少天？(用比例解)
<gadget id="calculator">20 * 28</gadget>
<output>560</output>

<gadget id="calculator">20 - 4</gadget>
<output>16</output>

<gadget id="calculator">560 / 16</gadget>
<output>35</output>

<result>35</result>