The equations they are referring to do not concern the nuclear aspects of an atomic bomb. They describe how to set up the geometry of conventional plastic explo

The equations they are referring to do not concern the nuclear aspects of an atomic bomb. They describe how to set up the geometry of conventional plastic explosives (different kinds of explosives that burn at different speeds) in just the right configuration so that the propagating shock waves interfere with each other to result in a spherical shock wave that pushes the core of the bomb (the radioactive part) towards a single fixed point at the center. If any of this is off even slightly, the forces will not be in balance and the core materials will shoot out of the weak point, rather than uniformly compress to bring the nuclear material to the critical point where the fission is self-sustaining.