There is 12 kg of water in bottle A, and 6 kg of salt water in bottle B. For the first time, pour part of the salt water in bottle B into bottle A, and then pour part of the salt water in bottle A back into bottle B for the second time. At this time, the concentration of brine in bottle A is 2.5%, and the concentration in bottle B is 7.5%. It is known that the concentration in bottle B is 10%, how many kilograms are poured back from bottle A for the second time?
甲瓶中有水12千克，乙瓶中有盐水6千克，第一次将乙瓶中的盐水倒一部分到甲瓶中，第二次再将甲瓶中的盐水倒回一部分到乙瓶中，此时甲瓶中盐水浓度为2.5%，乙瓶浓度为7.5%，已知原来乙瓶中浓度为10%，求第二次从甲瓶中倒回多少千克？
<gadget id="calculator">0.3 / 0.075</gadget>
<output>4</output>

<gadget id="calculator">10 / 100</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">6 * (1/10)</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">1.35 - (3/5)</gadget>
<output>0.75</output>

<gadget id="calculator">0.75 / 0.05</gadget>
<output>15</output>

<gadget id="calculator">12 - 15</gadget>
<output>-3</output>

<gadget id="calculator">4 + (-3)</gadget>
<output>1</output>

<result>1</result>