Two circular coils P and Q of 100 turns each have same radius of ....

Consider two coils as shown below:

Magnetic field due to P, ${B}_{P} = N\left(\dfrac{{\mu}_{0}{i}_{1}}{2r}\right) = 100 \times \dfrac{{\mu}_{0} \times 1}{2\left(\pi \times {10}^{- 2}\right)} = 2 \times {10}^{- 3}T$

Magnetic field due to Q, 

${B}_{Q} = N\left(\dfrac{{\mu}_{0}{i}_{2}}{2r}\right) = 100 \times \dfrac{{\mu}_{0} \times 2}{2\pi \times {10}^{- 2}} = 4 \times {10}^{- 3}T$

Therefore, ${B}_{net} = \sqrt{{B}_{P}^{2} + {B}_{Q}^{2}}$

$= \sqrt{20}mT$

Hence, $x = 20$