How many different methods are there to take 2 numbers out of 10 unequal natural numbers from 1 to 10 and sum them each time to make their sum less than 10?
从l～10这10个不相等的自然数中每次取出2个数求和，要使它们的和小于10，不同的取法有多少种．
<gadget id="calculator">1 + 2</gadget>
<output>3</output>

<gadget id="calculator">2 + 3</gadget>
<output>5</output>

<gadget id="calculator">1 + 3</gadget>
<output>4</output>

<gadget id="calculator">3 + 4</gadget>
<output>7</output>

<gadget id="calculator">1 + 3 + 5 + 7</gadget>
<output>16</output>

<result>16</result>