A force of 100N stretched an Elastic string to total length of 20cm,if an addition force of 100N stretches the string 5cm further.find the Natural length of the string.

You can assume that a force of x newtons always stretches the string by a constant amount. With the application of the second force of 100N it stretched 5 cm. So you can assume that the initial application of the first 100N stretched the string from its initial "natural" length to the 20 cm figure you were given. Can you now solve for the "natural" length?