Remark
The basic equation is y = mx + b
y = the total amount of money
m = The amount of money he save each month (it is a constant).
b = the y intercept which is his starting amount or 120 dollars.
x = the number of months (this is a variable).
After 3 months he has 840 dollars. That's y
840 = 3*m + 120 Subtract 120 from both sides
840 - 120 = 3m
720 = 3m
240 = m So he saves 240 every month.
Now the equation becomes.
y = 240 * x + 120
Savings after 24 months
y = Total
m = 240 dollars/month
x = 24 months
b = 120
y = 24*240 + 120
y = 5760 + 120
y = 5580
He had 5580 dollars after saving for 2 years (or 24 months total) This assumes that when he started, he took his birthday money and added a 1 month payment to it
