Answer:
Let β = v / c
proper time Δt' is measured in a frame at the same place in a given frame - the proper time is measured in spaceship II moving at .28 c
a) spaceship I is moving at (.6 - .28) c = .32 c relative to proper time
Δt = Δt' / (1 - β^2)^1/2 as measured by students in ship I
Δt = 50 / (1 - .32^2)^1/2 = 52.8 m measured by students
b) Δt = 50 / (1 - .28^2)^1/2 = 52.1 min time elapsed for observer on earth