There are several weights with the masses of 9g and 13g respectively. To weigh an object with a mass of 3g on the balance, how many such weights should be used at least (the weights can be placed at both ends of the balance).
现有质量分别为9g和13g的砝码若干个，在天平上要称出质量为3g的物体，最少要用多少个这样的砝码(砝码可以放在天平的两端)．
<result>7</result>