XSLT to copy an XML with change of values

We have a xml which needs to be transformed using XSLT. But, we have to add two attributes of a tag and place it in other tag.

In below XML compilation tag with column 'Reusable' has values contrib and max_available. We want addition of these two attributes and want it in the attribute postPosition of compilation tag with column "Usable".

Input

<?xml version="1.0" encoding="UTF-8"?>
<answers>
    <answer name="compile">
        <output>
            <compilations>
                <compilation column="Usable" contrib="0" max_available="0" postPosition="35000">
                </compilation>
                <compilation column="Reusable" contrib="100.00" postPosition="100.00" max_available="200">
                </compilation>
            </compilations>
        </output>
    </answer>
</answers>

Output :

<?xml version="1.0" encoding="UTF-8"?>
<answers>
    <answer name="compile">
        <output>
            <compilations>
                <compilation column="Usable" contrib="0" max_available="0" postPosition="300.00">
                </compilation>
                <compilation column="Reusable" contrib="100.00" postPosition="100.00" max_available="200">
                </compilation>
            </compilations>
        </output>
    </answer>
</answers>