Answer and explanation:
Assuming Edmund has $0
From Edmund, we know the toy costs $40(assuming it's in dollars)
Lucy gives him $40 and has $10 less($40-$10) to buy her own toy
Therefore Lucy had $40+$30= $70 before giving Edmund $40 for his toy.
Lucy currently has $70-$40 =$30, therefore needs $10 more to buy her own toy