The work done on the flagstone by the crane is equal to the product of the force applied and the distance it is moved in the direction of the force. In this case, the force is the tension in the rope of the block and tackle that is lifting the flagstone, and the distance is the height to which the flagstone is lifted.
The block and tackle system reduces the force required to lift the flagstone, while increasing the distance through which the rope must be pulled. The amount of force reduction depends on the number of pulleys in the system. Assuming that the block and tackle has a mechanical advantage of 4 (meaning that the force required to lift the flagstone is 1/4 of the weight of the flagstone), the tension in the rope will be:
Tension = Force required / Mechanical advantage
Tension = 2000 N / 4
Tension = 500 N
The work done on the flagstone by the crane is then:
Work = Force x Distance
Work = 500 N x 30 m
Work = 15,000 J
Therefore, the crane does 15,000 joules of work on the flagstone.