Simple telescope (lenses)

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In a simple telescope (converging lenses, long focal length objective and short focal length eyepiece), why is the distance between the objective and the eyepiece approximately equal to the sum of their focal lengths?

Sorry if this sounds like it is straight off a homework question, but well, it is. However, it really got me thinking about it.

I'm not looking for just a quick answer so I can get my points. I really want to understand why it works out this way. It seems logical enough, but I don't know the real reason why the distance between is equal to the sum of the focal lengths. Can someone explain why this is? Maybe a diagram/drawing? Thanks.

I know the equation for lenses in contact with one another, but is there an equation for lenses that are apart like in this case?