The length of an elastic string is 5 metre when the longitudinal tension is 4 N and 6 metre when the tension is 5 N. If the length of the string (in metre) is "2X" when the longitudinal tension is 9 N (assume Hooke’s law is valid) then the value of X will be :

Explanation

Let the original length of the string be L
Applying F=kx, we have 4=k(5-L)
5=k(6-L)
9=k(2X-L). From these equations x=5