Answer: Kristina sister's age = 15 years
Explanation:
LET the sister's age be rep as x
SO that Kristina = 4/5 of x
Given that Kristina's age = 12 , Then her sister's age can be found using The equation
4/5 x X= 12
4/5 X =12
X = 5 X 12 / 4
=60/4=15
Kristina's sister's age = 15 years.