Dear Padala,

For an observer inside the rocket, the mass observed by him will be the same as 100 kg as he is also travelling in the same frame of reference as the other man. However for an observer standing on the surface of the earth, the observed mass will be the relativistic mass. Relativistic mass is given by the formula:

                                                      Gamma= 1/(sqrt(1-(v^2/c^2)))

This will be the factor to be multiplied with the actual mass. Given v= 4.2*10^7, the factor gamma comes out to be 1.00994645407. Multiplying this with the actual mass,i.e,100 kg, we get the relativistic mass as 100.99 kg.