What is the Assumption Method?

The Assumption Method ( also known as the Supposition Method ), is a Singapore Math problem-solving technique where you assume an extreme situation to solve a question that you might use the Guess and Check method learnt in Primary 3.

Don’t get us wrong. Guessing and checking does work, and it’s a good start for children who need a bit of trial and error to understand certain Math problems.

However, if you find your child still trying to guess and check their answers for those difficult Math questions in the upper primary levels (e.g Primary 4 or 5 ), you might want to try teaching them a faster and more systematic method such as the Assumption Method.

Although this method does take a while to get used to, it’s gong to be time well-spent learning it, considering how much time and energy it’s going to save you.

Using assumption for Math problem solving

How does the Assumption Method work?

We start by making an assumption or guess about something ( See how the Guess and Check method and the Assumption is related? ).

Then, we’ll deduce what we know and how it links to the word problem step-by-step until we solve the question.

Before we learn how to do the Assumption Method, let’s talk about how to identify the kind of questions that we can use it to solve.

Examples of Assumption Method Questions

Here are some examples of Primary 3 to Primary 6 Math problem sum questions that we can solve with the Assumption Method:

Primary 3 and 4 Math:

There are 15 mosquitoes and pest busters somewhere in Singapore.

There are 54 legs altogether.

How many mosquitoes are there?

Primary 5 and 6 Math:

Iron Man made a total of 21 gadgets and masks for $3 340.

It cost him $190 to make each gadget and $140 to make each mask.

How many masks did he make?

How do we identify questions that use the Assumption Method?

Does your Math question have:

  1. 2 types of objects with something in common?
  2. the total number of objects and the total number of the common property?
  3. the individual number of each object missing?

If it does, chances are, you are looking at an assumption question.

Using the question on the left as an example, we have mosquitoes and pest busters and the thing that they share in common are legs.

We have 15 mosquitoes and pest busters and they have a total of 54 legs. Since this is a Primary 3 or 4 Math problem, the P4 students are expected to be able to deduce that a mosquito has 6 legs while a pest buster has 2.

We don’t know the exact number of mosquitoes or pest busters.

Steps for using the Assumption Method

Step 1:

ASSUME everything to be of the same type

Let’s assume that there are only pest busters.

Assumption method step 1
Step 2:

MULTIPLY to find the total value

Next, calculate the total number of legs that we have.

No. of legs of 1 pest buster x No. of pest busters in our assumption

= 2 x 15

= 30

Looks like we have 30 legs.

However, when we look at the question, we are supposed to have 54 legs.

Step 3:

Find the DIFFERENCE (The gap between what we have in our assumption and what’s given in the problem)

54 – 30 = 24

Subtract the total number of legs in the problem from the total number of legs in our assumption.

Looks like we are short of 24 legs. What shall we do to get more legs then?

Remember the mosquitoes and pest busters that were mentioned in the question?

Since we have been working with only pest busters, it’s time to bring in the mosquitoes!

Step 4:

Find the EFFECT of replacing 1 item with the other

Subtract the number of legs of 1 pest busters from the number of legs of 1 mosquito.

6 – 2 = 4

This means that replacing 1 pest buster with 1 mosquito is going to increase the total number of legs by 4.

assumption method step 2
Step 5:

REPLACE subjects until we close the gap

Find the number of replacements we need to make so that we can close the gap of 24 legs.

24 / 4 = 6.

This tells us that we need to replace 6 pest busters with 6 mosquitoes.

That’s how we know we have 6 mosquitoes.


assumption method step 3

And that’s how you do the Assumption Method!

The next time you see Guess and Check questions for Primary 4, try using the Assumption Method instead of the Guess and Check method and see how much faster it takes to get the answer.

Need more examples?

Check out this assumption method p5 question we did for our learners!

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