Amount of a substance left after a given time when the half-life is known

The same question, asked 2 times: 3 May 2023 S1*, 4 May 2023 S2. Every wording below, oldest first.

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Question 1
3 May 2023 S1
120 $If the half-life of substance is 5 years, then the total amount of the substance left after 15 years, when initial amount is 64 gms is$
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Question 2
4 May 2023 S2
127 $The decay rate of radium is proportional to the amount present at any time t. If initially 60 gms was present and half life period of radium is 1600 years, then the amount of radium present after 3200 years is$
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Question 1