Answer
$2.9\times10^{-4}T$
Work Step by Step
Model the jumper cable as a long straight wire. Use equation 20–6.
$$B=\frac{\mu_oI}{2\pi r}$$
$$=\frac{(4\pi \times10^{-7} T\cdot m/A)(65A)}{2\pi (4.5\times10^{-2}m)}$$
$$=2.9\times10^{-4}T$$
Taking a ratio, we see that this is about 5.8 times the strength of the Earth's magnetic field.