How many different two-digit numbers can be formed with the six numbers 1-6.
用1-6六个数字组成多少个不同的两位数．
<gadget id="calculator">5 + 4 + 3 + 2 + 1</gadget>
<output>15</output>

<gadget id="calculator">15 * 2</gadget>
<output>30</output>

<result>30</result>