Two resistors have resistances R_1 = (100 ± 3) \, and R_2 = (200 ±...

The equivalent resistance in series is given by:
$R_{eq} = R_1 + R_2 = 100 + 200 = 300 \, \Omega$

In addition, the absolute errors are added to find the maximum possible error:
$\Delta R_{eq} = \Delta R_1 + \Delta R_2 = 3 + 4 = 7 \, \Omega$

The percentage error in the equivalent resistance is:
$\dfrac{\Delta R_{eq}}{R_{eq}} \times 100 = \dfrac{7}{300} \times 100 = \dfrac{7}{3}\%$

Answer: $\dfrac{7}{3}\%$