∫ tension F= 1n(G object +G wheel), n 1=4, n2=5,
∴ Tighten the rope:
F A = 14(G object +G wheel), F B = 15(G object +G wheel),
∴F a > f b, b is more labor-saving;
∫ The weight of the moving pulley is the same, the weight and height of the suspended object are the same, W quantity =G wheel H, W useful =G object H,
The useful work, extra work and total work done by the pulley block are the same.
√η= W Total useful w,
The mechanical efficiency of the pulley block is the same, that is, η a = η b.
So, the answer is: b; η A = η B