The first row is 5 balls, and then each row is filled with 1 ball less than the previous row, and the 5th row is just full. How many balls can be put in total?
第一排放5个球，接着每一排都比上一排少放1个，放5排正好放满．一共能放多少个球？
<gadget id="calculator">5 + 1</gadget>
<output>6</output>

<gadget id="calculator">5 / 2</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">6 * (5/2)</gadget>
<output>15</output>

<result>15</result>