Centre of the cords divides the beam as 4 is of 6
b, the centre of the cords, stands to a as 2/3 of the segment b c
A short horizontal beam is drawn with two hanging weight-boxes and marked with the numbers 5, 5, 6, 4 and the letters a, b, c. Leonardo states a single proportion: just as 4 is two-thirds of 6, so b, the centre of the cords, stands with a as two-thirds of the segment b c, relating the position of the balancing point to the ratio of the loads.
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The centre of the cords set by the ratio of loads
The point b, taken as the centre of the two suspending cords, divides the beam so that its distance from a is two-thirds of the segment b c. This mirrors the numerical ratio marked on the diagram, placing the balancing point according to the unequal weights.
4 is two-thirds of 6
Leonardo grounds the geometry in a plain proportion of numbers: 4 is two-thirds of 6, and the same two-thirds relation governs how b, the centre of the cords, sits between a and c.