 Let "S" be the distance from the axle of the fixed block to the end of the rope, which is "A" where the input force is applied.Let "R" be the distance from the axle of the fixed block to the axle of the moving block, which is "B" where the load is applied.The total length of the rope "L" can be written as
where "K" is the constant length of rope that passes over the pulleys and does not change as the block and tackle moves.