the force of effort is given as
E = effort force = 280 N
m = mass of box to be lifted = 320 kg
g = acceleration due to gravity = 9.8 m/s²
force of load due to weight of box is given as
L = force of load = mg
L = 320 x 9.8
L = 3136 N
mechanical advantage for pulley system is the simply the ratio of the effort put in to the load being lifted.
mechanical advantage is given as
MA = load/effort
MA = L/E
MA = 3136/280
MA = 11.2