To solve this problem, we shall assume that the scales add or subtract a constant amount from the true weight.

 

The boy's weight, including the error, is recorded as 52 kg. When his sister steps on to the scales, the increase in weight, recorded by the scales, will be equal to her true weight.

 

Hence, the girl weighs 111 - 52 = 59 kg, and we deduce that the scales subtracts 3 kg from the true weight.

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