Answer
Sample with 20. ppb has the higher lead concentration.
Work Step by Step
20. ppb= $\frac{20\,\mu g}{kg}=\frac{20.\times10^{-6}g}{kg}$
0.0030 ppm= $\frac{0.0030\,mg}{kg}=\frac{0.0030\times10^{-3}g}{kg}=\frac{30.\times10^{-7}g}{kg}$
Sample with 20. ppb has the higher lead concentration.
