If a woman weighs 125 lb, her mass expressed in kilograms is x kg, where x is

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.