A long straight wire carries 16 A. At what distance from it is its field 40 µT?

Step 1. For $40\ \mu\text{T}$. $r = \dfrac{2 \times 10^{-7}\ \text{T m/A} \times 16\ \text{A}}{4 \times 10^{-5}\ \text{T}} = 0.08\ \text{m} = 8\ \text{cm}$.

Step 2. For $10\ \mu\text{T}$. The field is a quarter, so the distance is $4$ times: $4 \times 8\ \text{cm} = 32\ \text{cm}$.