How to solve hardest PSLE Math 2025 question easily

Question: This figure is made up of 3 equilateral triangle. What is the length AB?

Since it is equilateral triangle, all sides are the same. If we can solve 1 side of the triangle, we can solve this question. Therefore, there are 2 ways to solve this question: Algebra/Simultaneous Equation method, or Overlapped Shapes concept.

Algebra/Simultaneous Equation method

Generally, when I don't know how to solve a question, I label what I don't know

Why this is simpler in my opinion? Because labelling what you don't know, doesn't require you to think or solve anything.

Now, once we have labelled the unknown sides of the figure, we can create equations based on this:

Length AB = 18 + A

Length AB = A + B + 3

Length AB = C + 3

Length AB = 41 - B

From here, we don't need (C + 3) equation, because there is no way to solve for it. So we are left with:

Length AB = 18 + A

Length AB = A + B + 3

Length AB = 41 - B

Since AB is the same, we can create an equation like this:

18 + A = A + B + 3

And if we solve this:

A + B + 3 = 18 + A

B = 18 - 3 + A - A

B = 15

Since AB = 41 - B, that means AB is 41 minus 15 which is equal to 26cm

Overlapped Shapes concept

Whenever a shape is overlapped, the distance from the overlapped area is the same.

Therefore, the 18cm is also on the other side of the overlapped shape.

From here, you can calculate the right shaded triangle length

right shaded triangle length = 41 - 18 = 23cm

Therefore, the length of AB is the right shaded triangle length adds with 3cm

AB = 23 + 3 = 26cm

Summary

If you have done overlapped shapes questions before, this question is actually doable. However, if you have not, then it would be very hard.

However, if you were to use simultaneous equation and labelling the unknown lengths, it's much more doable in my opinion.

Therefore, everyone has their prefer methods in solving this kind of question.