Explanation
The mechanical advantage (MA) of a block and tackle pulley is equal to the number of times the effort force changes direction. An easy way to count how many times the effort force changes direction is to count the number of ropes that support the resistance which, in this problem, is 4. With a MA of 4, a 120 lb. effort force could lift \(120 \mathrm{lb} . \times 4=480 \mathrm{lb}\) resistance.