It has already been answered, but here is how to solve. C is the variable for cousin's age and S is for sister.
S+26=C
C•S=560
Substitute C in the first equation into the second one.
(S+26)•S=560
S^2+26S=560
S^2+26S-560=0
(S+40)(S-14)
S=14 or -40
Eliminate
-40 The answer is 14. Your sister is 14.
