More pedantically, the math you've been trained to do to find a velocity 's' as the sum of two velocities 'v' and 'u' in the same direction is s = v + u.

The real formula is s = (v+u)/(1+(vu/c^2)).

For small values of v and u, vu/c^2 is approximately 0, leaving you with the standard (Galilean) formula, but only as an approximation.

See Wikipedia for a discussion and formulae for vector addition: http://en.wikipedia.org/wiki/Velocity-addition_formula