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I'm stuck at this 2 questions for a while.

1. A mass suspended from a vertical spring is displaced vertically downwards by a distance A and released. It executes simple harmonic motion about its equilibrium position O. Given that time taken for the mass to move through a distance of 0.5A from the point of release is t, what is the period of the oscillation of the mass?

I drew the cosine curve and at 0.5A, the corresponding time is a unknown, but it is >T/8 where T is the period. How do I carry on from here?

2. Over a period of 6 seconds, a current is uniformly reduced from 150mA to 30mA. WHat is the charge that flows during this time?

The worked solutions is (150-30)6 = 720mC.

720-150-30 = 540mC which is the answer.

Why is this so? I would have thought that 720 is the reduction in charge? And why the subtraction of 150 and 30?

Thanks for the help in advance.