A magnet is suspended in such a way that it oscillates in the horizontal plane. It makes 20 oscillation per minute at a place where dip angle is `30^(@) and 15` oscillation per minute at a place where dip angle is `60^(@)`. The ratio of earth's magnetic fields at two places is

Explanation

we known that frequency ,
`v=(1)/(2pi)sqrt((MB_H)/(I)) or v prop sqrt(B_H) or v prop sqrt(B cos delta)`
` or B? cos delta prop v^(2) or B prop (v^(2))/(cos delta)`
`(B_1)/(B_2) = (v_1^(2))/(cos delta_1) xx ( cos delta_2)/(v_2^(2)) = (20 xx 20 xx cos 60^@)/(cos 30^(@) xx 15 xx 15 ) = (16)/(9 sqrt(3))`