Consider a solenoid of length L, N windings, and radius b (L is much longer than b). A current I is flowing through the wire. If the radius of the solenoid were doubled (becoming 2b), and all other quantities remained the same, the magnetic field inside the solenoid would

The magnetic field remains the same.

Explanation:

If a solenoid has length L, N windings, and radius b, then the magnetic field inside the solenoid is given by :

is magnetic permeability

If the radius of the solenoid were doubled and all other quantities remained the same, the magnetic field inside the solenoid would remains the same as it is independent of the radius of the solenoid.

d) the number of neutrons plus the number of protons

this is poiselle's equation from fluid mechanic in physics. mech eng