What design pattern does this?

I did this once a long time ago and followed a design pattern when I did. Now, I need to do it again, I don't really remember how I did it before, and I can't think of the pattern that helped me do it.

I have a class with a whole slew of variables/properties. Some are calculated based on the others, and there is all sorts of cross-calculating going on between these properties.

It's all fine when I first instantiate - all the values and the calculations work just fine. My problem is, when one value changes, I want all of the calculated values derived from it to update themselves based on the new value automatically. And I don't want to write each individual recalc manually if I don't have to - it just becomes a lot of overhead whenever this class gets updated or added to, trying to track down all of the places you need to propagate whatever change you're making.

I think you follow me.

Anyway, can anyone think of what pattern it is that makes this possible? I swear I used to know it. Getting old I guess.

// Like this...
class foo
{
    decimal A = 1233;
    decimal B = 42;

    decimal C = A / B; // I want this to update whenever
                       // the value of either A or B changes.
    decimal D = 123;
    decimal E = 321;

    decimal F = D + E; // I don't want this one to change when
                       // A or B or even C for that matter changes,
                       // and I don't wan to have to cycle through
                       // all of the calculated values that don't
                       // need to change just for find the few that do.
}