answer is correct need to figure out how its derrived

4. The core below is made of cast steel. Find the coil MMF required to get an air gap flux of 60 kilo-lines. The cross- sectional area of the center section and air gaps is 4 in. The remaining sections have a cross-sectional area of 6 in?. The air gaps are both 0.06 in. The lengths are ls=7 in, and I2 = 4 in. Ans. 1986 A-T 3,5 .12 Air Gaps N u. 3.5 Ain=12 l=22 A=4in2 elu H u 233 28 139 230 Al A 1/2 Cast Steel Goookline 4 in 15.000 Air Good 4in 233 147022 12564.26 45 3 F4103