A bookshelf is divided into upper and lower layers. If the upper layer is increased by (1/7) and there are 260 books in total, and the lower layer is increased by (1/7) there are 275 books in total. How much is the difference between the upper and lower layers?
一书架分上下层，如果上层增加(1/7)后共有260本，下层增加(1/7)后共有275本，上下层相差多少本？
<gadget id="calculator">275 - 260</gadget>
<output>15</output>

<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">15 / (1/7)</gadget>
<output>105</output>

<result>105</result>