Why is there no sea floor that is older than about 250 million years?

The oldest oceanic crust is about 260 million years old it is due to the process of subduction ,oceanic crust tends to get colder and denser with age as it spread off the mid ocean ridges . It gets so dense that it sinks in the upper mantle (subduction).

The hundred years' war

Well, a and b together would be negative. b alone would also be negative.