Form 4 Static

Let F be the tension in the upper rope
hence, for the max force acting on the device, we have
Fcos(30) = 900
i.e. F = 900/cos(30) N = 1039 N

Consider the fireman and packet as a system,
net ofrce acting on the system = [1039 - (65g + 10g)] N = 289 N
where g is the acceleration due to gravity, taken to be 10 m/s2

Use: net-force = mass x acceleration
289 = (65 + 10)a
where a is the acceleration of the system (fireman and packet)
solve for a gives a = 3.85 m/s2