First, every airplane has to be trimmed, so it flies straight (not up or down) when you let the controls go. Normally for that you set small trim fins on the elevator. On passenger planes you move the horizontal stabilizer. Normally that is done in the cockpit by turning a wheel forward or backward.
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Half-life of 8 days means, that half of it remains after 8 days. So: After 8 days it´s 64/2=32g, after another 8 days: 32/2=16g and another 8 (then it´s 24) : 16/2=8g.