Let the amount that Brandy paid be b, the amount that Katie paid be k, and the amount that Stephanie paid be s.
b=2k
s=k+3
b+s+k=56
Substituting and doing the equation only in terms of k, we get:
2k+(k+3)+k=56
4k+3=56
4k=53
k=13.25
Katie paid $13.25.
Hope this helps!