The force exerted by the magnetic in terms of the magnetic field is,

Where B is the magnetic fied strength and F is the force.
Thus, if the magnetic A has twice magnetic field strength than the magnet B,
Then,

Thus, the force exerted by the magnet B is,

Thus, the force exerted by the magnet B on magnet A is 50 N.
The force exerted by the magnet A exerts on the magnet B is exactly 100 N as given.
Hence, the option B is the correct answer.
Wildlife researcher starts from a and then reaches b, he turns towards north 40 degree to move towards c.
Total displacement is ac
Total horizontal displacement = 4+4 cos40 =7.06 km
Total vertical displacement = 4 sin40 =2.57 km
Total displacement
= 7.51 km
I'll go ahead and answer the ones here without an answer. For reference, the half-life formula is <em>final amount = original amount(1/2)^(time/half-life)</em>
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4) 12.5g
x = 100(1/2)^(63/21)
5) 50g
3.125 = x(1/2)^(0.1/0.025)
6) 500g
x = 4000(1/2)^(525/175)
7) 0.24g
0.06 = x(1/2)^(11430/5730)
8) 125g
x = 1000(1/2)^(17100/5700)
Hope this helps! :)