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B 30 kg because
One quarter =1/2
Kg=60
1/2 of Kg
1/2*60
30 kg
The amount left of a given substance can be calculated through the equation,
A = (A0) x 0.5^n/h
From the given scenario,
A/A0 = 0.75 = 0.5*(60/h)
The value of h from the equation is 144.565 minutes.