Correct Answer - 18
Let number of loops in steel are `P` and in aluminium number of loops are `Q`.
`:. (P)/(2l_(s)) sqrt((T)/(mu_(s))) = (Q)/(2l_(Al)) = sqrt((T)/(mu_(Al))) :. (P)/(Q) = (4)/(3) …..(1)`
For minimum frequency `P = 4`
`f=(4)/(2xx(80)/(100))sqrt((40)/(10^(-6)xx7.6xx10^(3)))`
`f = 180 Hz`