Look up the formula for the inductance L of a solenoid. It is something like
L = mu* N^2 *A/l
W here l is the length and N is the number of turns. A is the cross sectional area. Mu is the permeability of free space
Use the induced emf for known current change rate to get the inductance.
V = L * dI/dT
Then use the solenoid formula to get the area A, which is pi*R^:2
An inductor in the form of a solenoid contains 440 turns and is 15.8 cm in length. A uniform rate of decrease of current through the inductor of 0.421 A/s induces an emf of 175 µV. What is the radius of the solenoid?
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