The metric staircase. Every step is ten, so converting is just sliding the decimal point.
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Inches, feet, pounds, cups. No pattern to lean on — so go down to the smallest unit and back up.
Count on to the next whole hour, then the hours, then the leftover minutes. Works every time.
The two get mixed up constantly. One is the walk around the edge, the other is the space inside.
An L-shape is just two rectangles. Slice it, do each piece, add them up.