Idea 15of 21Builds onWeek 0 comes first
The rule as 5x + 15
How can a few numbers and one letter hold a whole pattern?
Today you'll discover
- what the letter x stands for
- where the 5 and the 15 come from
- how to check an expression against the table
Think firstWeek 3 in the table is $30
Which of these works out to Suzie's $30 for week 3?
Decide first, then press Next. Match each number in the expression to the story.
A savings table of weeks 0 to 8 and money $15 to $55. Above it, the expression 5x + 15, with 5x labelled $5 times weeks and 15 labelled the $15 at the start. The last step asks for Jimmy's expression.
See the whole ideathe big idea and the key word, in one place
The big idea
In 5x + 15, x counts the weeks, 5 is the change and 15 is the start.
Shrinking patternsA shrinking pattern takes away instead. Start at 100 and lose 10 each week, and the expression is 100 − 10x.
Easy to get wrong5x means 5 times x. When x is 4, 5x is 20, not 54.
Key words
- algebraic expression
- numbers, a letter and signs that show a rule, like 5x + 15
- variable
- a letter, like x, that stands for a number that can change
Ready to use it?
- Write an expression for a savings pattern.
- Use an expression to find the money for any week.