(i) theory suggests that no object can travel faster than light;
the 1.7c is not the speed of a physical object;
so is not in violation of the theory;

(ii) recognition that v is negative relative to ux;
use of \(\frac{{0.85c + 0.85c}}{{1 + \frac{{{{\left( {0.85c} \right)}^2}}}{{{c^2}}}}}\);
(0.9869c≈) 0.99c;
Accept first marking point implied in the second marking point.

(i) \(\gamma \)=1.89;
interval on Earth = \(\gamma \)×interval on spaceship;
(interval on Earth 1.90×8 years=)15 years;
Award [3] for a bald correct answer.

(ii) time interval measured by observer on Earth is not proper because the time interval between the two events is not measured at same place/not the shortest time;

(iii) observer on Earth thinks spaceship has travelled for 15 years;
so distance is 0.85c×15=12.8≈13 ly;
Award [2] for a bald correct answer.

or

the spaceship observer observes the distance moved by the Earth=0.85c×8.0yr;
proper distance=1.90×0.85c×8.0 yr=12.9≈13ly;
Award [2] for a bald correct answer.

(iv) Earth is at a distance of 4×0.85c=3.4ly when signal is emitted;
signal reaches Earth in time T where cT+3.4=0.85cT;
T=22.7≈23 years;