a wire of resistance 20 ohm is stretched in order to double its length. so i want to know what is the new resistance?

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For deciphering what will happen to the area of cross section in such questions you can use a simple trick that is A1L1=A2L2 . In this case let A1=A and L1=L (i.e. assume the area and length of the wire to be these before doubling its length), now you know that L2=2L1=2L an you have to find out A2 , simple just substitute these values into the equation, so you will have AL=A2(2L) => A2 = AL/2L =A/2. thus you can see that the area of cross section after stretching the wire to twice its length will become half of its original value . This method always works , you just have to make a small assumption for A1 and L1. Hope that helps.