Least number of complete years for a sum at r% compound interest to more than double or treble

The same question, asked 2 times: CDS 2 2019*, CDS 1 2025. Every wording below, oldest first.

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Question 1
CDS 2 2019
44 $What is the least number of complete years in which a sum of money put out at 40% annual compound interest will be more than trebled ?$
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Question 2
CDS 1 2025
49 $What is the least number of complete years in which a sum of money put out at 20% compound interest (compounded annually) will be more than doubled ?$
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