The average circumference of the magnetic circuit is the circumference of a circle with a radius of 15cm, and the cross-sectional area of the magnetic circuit is the area of a circle with a radius of 1cm.
φ=BS=μHS=μ (inch/liter) s = 4π x10-7x4.5x1500/(2π x 0.15) xπ 0.012.
=2.826X 10-6Wb