How can i decease one variable by 0.1; for increment of other variable by 0.5..?

Say I have two variables(x&y). now for the increment of 0.5 in X; variable Y should decrease by 0.1. I have written this with IF condition. but it's lengthy. the code is as follow:

**************************************************

if temp >= 40 
    Iref=.9*AH;
end
    if temp >= 40.5 
        Iref=.8*AH;
    end
            if temp >= 41 
                Iref=.8*AH;
            end
                if temp >= 41.5 
                    Iref=.7*AH;
                end
                    if temp >= 42
                        Iref=.6*AH;
                    end
                if temp >= 42.5 
                    Iref=.5*AH;
                end
            if temp >= 43 
                Iref=.4*AH;
            end
        if temp >= 43.5
            Iref=.3*AH;
        end
    if temp >= 44 
        Iref=.2*AH;
    end    
if temp >= 44.5 
    Iref=.1*AH;
end

*******************************************************

please suggest some other short way to do this.

Answer 1

[Matt J]

   Iref=(  (-0.1/0.5)*(temp-40)+0.9) *AH;

   Iref=ceil(  Iref/(0.1*AH)  )*0.1*AH;

[Darshan Joshi]

it decreases constantly like a line. I want the decrement in iref after every 0.5 increment in temp. I need output like this(output of the code written in question) :

[Matt J]

I've added a second line to discretize the output. Your plot however, does not match your code if AH is constant. Your code gives piecewise constant output whereas your plot is piecewise linear.