The graph shown represents John’s savings account balance this year. Based on the graph, how much money does John save each month on average?
Let’s look closely at the graph. The 𝑦-axis values represents the savings account balance in thousands of dollars and the 𝑥-axis tells us the month that we’re talking about. Our initial point occurs at month zero. At month zero, John already had a balance of 25000 dollars. We can tell this because the point falls halfway between the 20 and the 30 on the 𝑦-axis. We have another point given to us at month three. Month three occurs at the 40 on the 𝑦-axis, which tells us that by month three he had 40000 dollars in his savings account.
If we look at the graph carefully, we notice that at month one, the line crosses the 30. So we can say that at month one, the balance was 30000 dollars. We’re considering how much John saves each month. And that means we want to know the increase month by month.
From month zero to month one, he added 5000 dollars. We also see that there was an increase of 10000 dollars from month one to month three. A jump of 10000 dollars over two months has an average of 5000 each month. We can also see this on the graph itself. Since our 𝑦-axis is going up in tens of thousands, if we draw a line halfway between 30 and 40, halfway between 40 and 50, we can see an increase of 5000. At month four, we’ve gone from 40000 to 45000. At month five, we’ve gone up from 45000 to 50000, an addition of 5000 dollars each month.
Based on the graph, John saves 5000 dollars per month on average.