From the 10 natural numbers 1 to 10, take two different numbers each time so that their sum is a multiple of 3. How many different ways are there.
从1～10这10个自然数中，每次取出两个不同的数，使它们的和是3的倍数．共有多少种不同的取法．
<gadget id="calculator">5 + 10</gadget>
<output>15</output>

<result>15</result>