Answer:
Step-by-step explanation:
I take it that this is some sort of ratio. Start by solving the brackets on the left.
Brackets: 1/3 - 1/10
Brackets: 10/30 - 3/30
Brackets: 7/30
Brackets^2: 49/900
Brackets^2 - 1/5: 49/900 - 180/900
Brackets^2 - 1/5: -131/900
(2/5)^2 = 4/25
The way this read, it should be

Which when you invert and multiply becomes

which finally becomes

The answer for this question is 12 magnets. Hope this helps!
That would be 1/4 * 28 = 7 ins^2