It doesn't say if that's 120 ship years or Earth years. I'll assume ship years as that makes the most sense in context.
Assuming constant acceleration to the 1/2-way point, flip, deceleration, and using http://www.projectrho.com/public_html/rocket/slowerlight3.ph... :
T = (c/a) * ArcCosh[a*d/(c^2) + 1] (given acceleration and distance) 60 years = (c / a) * arccos(a * 30 ly / (c^2) + 1) 1893456000.0 = (3E8/a) * arccos(a * 3.15576 + 1)
a = 0.14748 m/s^2 or about 1.5% g
t = (c/a) * Sinh[a*T/c] = 1.15 * ship time ("proper time")
v = c * Tanh[a*T/c] = 0.73 * speed of light
It doesn't say if that's 120 ship years or Earth years. I'll assume ship years as that makes the most sense in context.
Assuming constant acceleration to the 1/2-way point, flip, deceleration, and using http://www.projectrho.com/public_html/rocket/slowerlight3.ph... :
Using Wolfram Alpha to solve - https://www.wolframalpha.com/input/?i=1893456000.0+%3D+%283E... Time elapsed on Earth is only a bit longer than ship time: Final velocity at flip is: