The length of a rubber cord is ℓ1  metres when the tension in it is 4N and ℓ2  metres when the tension is 5N. Then the length in meters when the tension is 9N is

Let 'ℓ'  be the original length and ℓ3  be the length of the wire when the tension is 9N. We know e∝F ⇒ (ℓ1−ℓ)∝4.....(1),(ℓ2−ℓ)∝5.....(2)(ℓ2−ℓ)∝9.......(3)From 'l'(1)&(2)ℓ1−ℓℓ2−ℓ=45⇒5ℓ1−5ℓ=4ℓ2−4ℓℓ=5ℓ1−4ℓ2............(4)From (1)&(3)ℓ1−ℓℓ3−ℓ=49    Put  is above equation and we get ℓ3=5ℓ2−4ℓ1