Lets label the variables...
x=Claudia's age now
y=Claudia's dad's age now
Lets write a system...
7x=y (7 of Claudia's age are equivalent 1 of her father's)
x+20=[tex] \frac{y}{2} [/tex] (Claudia plus 20 years is equal to half of Claudia's father's age now)
Now solve the system using the elimination of substitution method.
Hope that helps :)