arithmetic operation in XSLT

Question

How can I find the greatest 2 numbers out of 3 numbers and perform some arithmetic operations on them like in following example?

<root>
    <num>10</num>
    <num>12</num>
    <num>8</num>
</root>

For the above input the xslt code should display "10 + 12 = 22" and "average = 11".