The total cost of buying 3 kg of apples and 5 kg of pears is 27 yuan, and the total cost of buying 1 kg of apples and 1 kg of pears is 7 yuan. How much is the cost of buying 2 kg of pears?
买3千克的苹果和5千克梨共花费27元，买1千克的苹果和1千克梨共花费7元，买2千克的梨需付多少元．
<gadget id="calculator">7 * 3</gadget>
<output>21</output>

<gadget id="calculator">27 - 21</gadget>
<output>6</output>

<result>6</result>