This article demonstrates one method for converting from decimal to binary. This conversion is often used as a first step in converting from decimal to any other representation.
This is a quick method to convert a decimal number into a binary number.
The binary number is constructed from right to left.
Convert 99 to binary:
99 is 1100011 in binary.
Since this is meant to be done in one's head, maybe the ones and zeros could be kept track of with finger positions. Keep fingers by sides or on the table in front of you, slightly off the surface. Since it's calculated from right to left, the first binary digit would be represented by your right pinky and each additional digit would move to the left. If it's a 1, put your fingertip down. If it's a 0, put your finger down, but curl the finger.
Source: Art of Memory, https://artofmemory.com/wiki/How_to_Convert_Decimal_Numbers_to_Binary
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