From the 10 natural numbers 1 to 10, take two different natural numbers each time so that their sum is a multiple of 5. 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自然数，使它们的和是5的倍数．一共有多少种不同的取法．
<gadget id="calculator">2 + 4 + 3</gadget>
<output>9</output>

<result>9</result>