When a mother was weighed holding her baby, the scales read $78\;\mathrm{kg}$.

When the nurse was weighed holding the baby, the scales read $69\;\mathrm{kg}$.

When the nurse and the mother stood on the scales together, the scales read $137\;\mathrm{kg}$.

What was the weight of the baby?



If you liked this problem, here is an NRICH task that challenges you to use similar mathematical ideas.