Question 833425
Let C represent Catherine
Let R represent Criselda
12+C + 12+R = 54
C = 2/3R
substitute the value of C
12 + 2/3R + 12 + R = 54
subtract 24 from each side
2/3R + R = 30
Add Rs
5/3 R = 50
Multiply each side by 3 to get rid of the fraction
5R = 150
Divide by 5
R = 30
.
Let's check.
Catherine's present age is 2/3 of Criselda's (2/3 * 30 = 20)
In 12 years, they'll be 22 and 32.  Added together, that's 54.
Success!