1. The problem statement, all variables and given/known data Rest length of Ship B is twice that of Ship A. Ship A travels at 0.5c relative to a "fixed" observer. Ship B travels at such a speed that the same "fixed" observer measures Length B = Length A. How fast is Ship B traveling? 2. Relevant equations All given in statement 3. The attempt at a solution I have no experience with Latex, so pardon my clumsy equations. I know that the answer is that Ship B travels at 0.9c. But when I go through the math, I get things wrong. 1) BetaA=0.5 2) LA = gammaA * L0A (Length A = gammaA * Rest length A) 3) LB = gammaB * L0B. But 4) L0B=2 * L0A (rest length B = twice that of A) 5) LB = LA (length B is measured to be same as that of A) 6) So LB = LA = gammaB * (2 * L0A) Combining eqns 2) and 6), I get gammaA = 2 * gammaB. But is should be gammaA = 0.5 * gammaB. I'm clearly making a bad conceptual mistake. Can you point it out to me?