Math problem trying to make a progressbar in project zomboid

Question

I am making a mod for project Zomboid and I cant seem to figure out a math problem, so the value I am getting ranges from 0 to 1 and I want my progress bar to start at the max width and then descend as the value is increasing.

The first one was easy I got a value between a 100 and 0 so how do this with a value starting at 0?

I tried searching this on google but I am really bad at math and could not find an answer.

function panel:render()
    self:drawRectBorder(30, 30, self:getWidth() - 1, 50, 1.0, 1.0, 1.0, 1.0);
    --print((bt_core.player:getBodyDamage():getOverallBodyHealth() / 100) * self:getWidth());
    self:drawRect(31, 31, (bt_core.player:getBodyDamage():getOverallBodyHealth() / 100) * self:getWidth() - 3, 48, 1, 0, 0, 1);

    self:drawRectBorder(30, 110, self:getWidth() - 1, 50, 1.0, 1.0, 1.0, 1.0);
    print(bt_core.player:getStats():getFatigue());

    if bt_core.player:getStats():getFatigue() == 0 then
        self:drawRect(31, 111, self:getWidth() - 3 , 48, 1, 0, 0, 1);
    else
        self:drawRect(32, 111,bt_core.player:getStats():getFatigue() / (self:getWidth() - 3) , 48, 1, 0, 0, 1);
    end
end

Answer 1

To get variable in range 100..0 from variable in range 0..1, you can use y = 100 - x*100