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A magnetic compass needle oscillates 30 times per minute at a place where the dip is 45°, and 40 times per minute where the dip is 30º. If B 1 and B 2 are respectively the total magnetic field due to the earth at the two places, then the ratio B 1 / B 2 is best given by :

1. 1.8
2. 2.2
3. 0.7
4. 3.6

Correct answer: 0.7

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