All the while I have been following MOE method, and teaching base on their textbook. Hence, solving such questions using modeling is the easiest and quick.
Many parents always tried to "reverse engineering" or relate to their kids using Algebra. Unfortunately, such question is not design that way. For the time given during exam, modeling will be the fastest and lesser mistake.
So, my advice to all parents if they want to teach their kids, it's better to re-learn their method and coach them base on what they are taught in School. They will be happier and able to absorb better.