Kelly started with 90 dollars in her savings jar. Every week, Kelly spends 9 dollars from her savings jar at the grocery store. Represent this situation with a table, a graph, and an equation.
Let’s start by representing this situation with a table. Start by finding the amount in the savings jar after 0, 1, 2 and 3 weeks.
Time in weeks Number of dollars in the savings jar
0
1
2
3
Nice!
Plot the points below on the coordinate plane to represent the amount in the savings jar after 0, 1, 2, and 3 weeks.
Time in weeks Number of dollars in the savings jar
0 90
1 81
2 72
3 63
Great job!
We can connect the points you just graphed with a line. To find the equation that represents this linear function, you’ll need the rate of change and the initial amount in the account. Start by finding the rate of change. In other words, by how much does the amount in the savings jar change each week?
t
y
0
1
2
3
4
5
0
9
18
27
36
45
54
63
72
81
90
Time in weeks
Number of dollars in the savings jar
Rate of change:
The rate of change is -9 dollars per week, as Kelly spends 9 dollars from her savings jar each week.
Now, we can find the initial amount in the account, which is $90.
Therefore, the equation that represents this situation is:
y = -9x + 90
where y represents the number of dollars in the savings jar and x represents the number of weeks.