F9RK. Mgmt. 19, the task 5

The seller is blocked and sales of the product are suspended

5. The body weight of 18 kg is suspended from the spring, the upper end of which is fixed. At the same time the spring length of 10 cm. When it is suspended body weight 30 kg, its length becomes 12 cm. Calculate the work that makes the external force in tension springs 10 to 15 cm. What kind of work does with the elastic force?