A coil of copper wire (radius-r, self inductance-L) is bent in two concentric turns having radius r/2. The self inductance now is

magnetic field due to a coil is B=μ0ni2r  ,   flux linked with the coil is  Ï•=nAB = nπr2μ0ni2r=μ0n2πr2i=Li , so L∞n2r , here n increases by two times and r decreases by 2 times ,   so  L lincreases by two times