There are 1 gram, 3 gram, and 9 gram weights each. How many different weights can be weighed by choosing one or several of them and placing them on one side of the balance.
有1克、3克、9克的砝码各一个，选择其中的一个或几个放在在天平的一边可以称出多少种不同的重量．
<gadget id="calculator">3 + 3 + 1</gadget>
<output>7</output>

<result>7</result>