marginal cost

Take the derivative..

I don't get it? MC=change(Total cost)/change(Q)

SO that has to be: (200+5Q)/Q = 200+5?

MC is the cost of producing one extra unit. It should be known that dTC/dQ is determining the change in TC when raising Q by one unit. Thus, dTC/dQ=MC(Q) by definition.

Please remember that the slope of a linear function (e.g. this linear demand function) is given by Δy/Δx (ΔTC/ΔQ) but that is exactly the same as the derivative of the function.

What you're detemining in #2 is ATC (average total cost), although you made a mistake. ATC=200/Q+5

Ohhh... so in this case: TC=200+5Q I CAN actually - without calculating - see that the b=5?

Yes - in this case (what is b though?). If you have a slightly more complicated TC function, say, TC=2√(Q), then it might be more fun to find the derivative. But in the case with a linear curve it is always very easy to find MC.

lol yea I could c that. thnx a lot! (I just totally forgot abt derivativing) 

...a rule of thumb when studying economics: never forget about mathematics!

pretty gud idea! :)