Hi. I'll keep this brief and to the point.

I was teaching my sister, who's in Grade 5, math and I encountered the following problem:

Jack is 1/3 of Alex's age. In 12 years, he will be 2/3 of Alex's age. How old is Jack now?

Ans: 4 years

Of course using Algebra, I could solve it within seconds. Problem is, they do not teach Algebra in the 5th grade here, in Singapore. The standard method for solving these types of problems in Elementary school is by drawing models, based on the given fractions, then manipulating the models to find the answer. However, my knowledge of the model method is, at best, extremely rusty. I would appreciate if someone can tell me how to solve this problem without the use of algebra. Thanks!