The space shift Enterprise 1 is moving directly away from earth at a velocity that an earth-based observer measures to be +0.66c. A sister ship, Enterprise 2, is ahead of Enterprise 1 and is also moving directly away from earth along the same line. The velocity of Enterprise 2 relative to Enterprise 1 is +0.47c.

What is the velocity of Enterprise 2, (in terms of c) as measured by the earth-based observer?