The magnetic field within the turns of toroid is B=2πrμ0NI where, N= number of turns, I= current in loops and r= radius of each turn Here, N1=200, N2=100,r1=40 cm,r2=20 cm and current I is same, then B2B1=2πr1μ0 N1I×μ0 N2I2πr2 Substituting the given values in the above relation, we get B2B1∴B1:B2=( N2N1)(r1r2)=(100200)(4020)=2×21=1=1:1