Answer
The strength of the second magnetic field is $~81~mT$
Work Step by Step
We can write an expression for the initial magnetic field:
$B_1 = \frac{V_1~D}{R~I}$
We can write an expression for the second magnetic field:
$B_2 = \frac{V_2~D}{R~I}$
We can divide $B_2$ by $B_1$:
$\frac{B_2}{B_1} = \frac{\frac{V_2~D}{R~I}}{\frac{V_1~D}{R~I}}$
$\frac{B_2}{B_1} = \frac{V_2}{V_1}$
$B_2 = \frac{V_2}{V_1} \times B_1$
$B_2 = (\frac{2.8~\mu V}{1.9~\mu V})~(55~mT)$
$B_2 = 81~mT$
The strength of the second magnetic field is $~81~mT$
