D = 20 + C
C = 5 + L
We have three equations and three unknowns, therefore we are able to solve the problem. Let's substitute one equation into another until we have one equation with one unknown.
0.5*C = 5 Now divide both sides by 0.5
C = 10
Use this information to solve for L
L = 0.5*C = 0.5*10 = 5
Now plug these values into the equation for D.
If Leah and Lily are two different people, there are too many unknowns to solve this problem.