Using a Known Fact

Using a Known Fact: What is it?

In this addition and subtraction strategy students use a known fact that is closely related to the unknown fact to help them solve the equation.

Overview

In the example from the video, 6+5 is unknown. The student thinks about a closely linked known fact, 6+4 =10, then adjusts the initial equation by one, and therefore the answer by one which would make 11.

Commonly used known facts may include doubles, such as 6+6 or 5+5 to solve a problem, and adjust their solution accordingly or combinations of 10 (6+4, 7+3 etc.) to arrive at the answer.

Supporting Students Using a Known Fact:

Use tools like the rekenrek to model student thinking when one fact is derived from another.

Modelling 'using known fact' strategy using a rekenrek when solving word problems.

Question: You have 14 gummi bears. You eat 5. How many do you have left?

As students begin to reason with the numbers mentally, their thinking can be shown in written form.

Example:

Student: I know 5+7 is 12 because it is the same as 5+6, which is 11, and 1 more

Teacher: 5+7=5+6+1

(Lawson pg. 39)

Where to Next?

As students build on their strategies for working with numbers in more and more flexible ways, they can begin to develop other strategies such as decomposing one or both addends to find anchors of five, for example 6+7 =(5+1) +(5+2)and then onto up and over 10, for example 7+5=(7+3) +2.

Games & Activities to Support 'Using Known Facts'

Make 10 Concentration

In this variation of the classic ‘concentration’ or ‘memory’ game, students make a match by finding 2 cards that add to 10. (Lawson; pg. 172)

*This game supports known facts for combinations of ten.

Addition War

A modified version of the traditional ‘war’ card game. This game provides a wider range of 'known facts' to be used to determine the sum of the two cards flipped. (Lawson; pg. 176)

I Spy

In this game, students challenge each other to find different ways of making the same sum.

Looking to learn more? Check out the recorded session!

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