College Physics (4th Edition)

$L = 13.1~m$
Let $L_0 = 30.0~m$ We can find the length measured according to an astronaut on the other spaceship: $L = L_0~\sqrt{1-\frac{v^2}{c^2}}$ $L = (30.0~m)~\sqrt{1-\frac{(0.90~c)^2}{c^2}}$ $L = 13.1~m$