Finding the unknown (4 operations) I

Lesson

Practice

Lesson

Finding a missing number

To solve a problem with missing values, we can use a range of strategies, such as those we used when solving addition and subtraction problems with missing numbers, as well as those where we had multiplication and division problems with missing numbers.

In Video 1 we look at solving problems with one step, using a variable (letter of the alphabet) as the unknown number. Once we work out what value it needs to be, we can replace the variable with the number. Let's see how we do this, using a number line, counting to the nearest ten, and place value.

Two-step questions

If our problem has two parts to it, we need to work out what we can solve first to help us find the missing number. Once we have solved one side of our number problem, we now have a one-step problem. Are there any other ways we could solve these? Yes! In Video 2, we'll see two different ways to solve a two-step problem. Which one did you prefer?

Remember!

There are often several ways to work out the missing number, so you may find it useful to try different strategies. The letter, or variable, is there until we work out the missing value.

Worked examples

Question 1

What must $p$p equal to make the following equation true?

$16+p=19$16+p=19

QUESTION 2

What must $j$j equal to make the following equation true?

$53-j=34$53−j=34

QUESTION 3

What must $w$w be equal to so that $w\times25=20\times10$w×25=20×10 is true?

Outcomes

6.PA2.01

Demonstrate an understanding of different ways in which variables are used (e.g., variable as an unknown quantity; variable as a changing quantity)

6.PA2.04

Determine the solution to a simple equation with one variable, through investigation using a variety of tools and strategies (e.g., modelling with concrete materials, using guess and check with and without the aid of a calculator)