Answer didnt check with back of book

hey,

I'm not getting the answer matching the back of the book for these questions, can you show me how to do it with your work?

A long wire is known to have a radius greater than 4.0 mm and to carry a current uniformly distributed over its cross section. If the magnitude of the magnetic field is 0.285 mT at a point 4.0 mm from the axis of the wire and 0.200 mT at a point 10 mm from the axis, what is the radius of the wire?

and

A long solenoid (n = 1200 turns/m, radius = 2.0 cm) has a current of a 0.30 A in its winding. A long wire carrying a current of 20 A is parallel to and 1.0 cm from the axis of the solenoid. What is the magnitude of the resulting magnetic field at a point on the axis of the solenoid?

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Can you show us what you tried and what answers you got (and how) ?

For the first question, Im assuming you calculated the magnetic field using
$$\int \vec{B} \vec{dl} = \mu_0 I_{\text{enclosed}}$$

Did you keep in mind that, at least for the 4mm radius, the enclosed current is NOT the total current?

Finally find the right latex code, sorry :p
$$\oint \vec{B} \cdot \vec{dl} = \mu_0 I_{\text{enclosed}}$$

(Why can't I edit my post??)