Put some water in a glass bottle, double the amount of water, and weigh 5 kg; add 4 times the original amount of water, and weigh 9 kg again. How many kilograms does the water weigh?
玻璃瓶里装一些水，把水加到原来的2倍，称的重5千克；把水加到原来的4倍，再称重9千克．原来水重多少千克？
<gadget id="calculator">9 - 5</gadget>
<output>4</output>

<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<result>2</result>