The fact that the collision is not an elastic one means that some kinetic energy turns into other form(s). Therefore the quantity of kinetic energy before and after the collision is different.
Note that after this collision the bull and the man have different speeds, so they don't move as a whole.
For a mass `m` moving with a speed `V,` its kinetic energy is `(m V^2)/2.` So after the collision the bull has kinetic energy `(500*10^2)/2 = 25000 (J)` and the man has `(70*5^2)/2 =875 (J).` Their total kinetic energy is `25000 J + 875 J = 25875 J.` This is the answer.
Just in case, compute the initial kinetic energy of the system bull+man before the collision. It is `(500*15^2)/2 + 0 =56250 (J),` so a huge amount of energy was "used" to damage the man (and the bull).