So I'm reading the book on physics in the chapter: Inductance. The questions is: The long Solenoid turns/cm and carries a i = 1.5 A, its diameter D is 3.2cm. At its centre we place a 130- turn closely packed coil C of diameter d = 2.1cm. etc etc My question is, in the solution the N is 130, but its 220. And I noticed 2 N's. N and n. N is number of turns which should be 220 but its actually 130, and n is 220. Shouldn't it be N1 and N2? Easier to understand, I completely skimmed it through and didn't see the lower case n.