The first thing you should know to solve this problem is the conversion of pounds to kilograms:
1lb = 0.45 Kg
We can solve this problem by a simple rule of three
1lb ---> 0.45Kg
125lb ---> x
Clearing x we have:
x = ((125) / (1)) * (0.45) = 56.25 Kg.
Answer
her mass expressed in kilograms is 56.25 Kg.