decrease number?

I have several hundred lines like the one below:

<line x1="20"  y1="45"  x2="820" y2="45"  stroke-width="1" stroke="#c0c0c0" />

I want to change them like this:

<line x1="10"  y1="35"  x2="810" y2="35"  stroke-width="1" stroke="#c0c0c0" />

The "ten" place holder needs to decrease by one.
x1="20" y1="45" x2="820" y2="45"

Can someone point me to something that would show me a fast way to do this?

I am thinking some how I should be able to use CTRL-X in a search and replace but I don't know how to do it. I am guessing '\=' would be used. I really don't know how to put this together.

Thanks

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