Refer to figure.
In order to maintain the balance, the moment produced by the mass A should be equal to the moment produced by the masses B and C in order to cancel each other out and result in a total moment of zero.

• The moment of the mass A will be Moment_A = mass_A​​​​​​ x distance_A = 309 kg x 6 m = 1854 kg m
• The moment of the mass B will be Moment_B = mass_B x distance_B = 36 kg x 5 m = 180 kg m
• The moment of the mass C will be Moment_C = mass_C​​ x distance_C = 120 kg x distance_C

To achieve balance, the following should apply:

Moment_A = Moment_B + Moment_C
1854 kg m = 180 kg m + 120 kg x distance_C
120 kg x distance_C = 1854 kg m - 180 kg m = 1674 kg m
distance_C = 1674 kg m / 120 kg = 13.95 m

Therefore, mass C should be at a distance of 13.95 m from the datum. The distance between masses A and C will be 13.95 m + 6 m = 19.95 m.

Note: A moment is defined as the product of a mass and its balance arm (distance from a reference point).

• Moment = Mass × Balance Arm