Answer:
The option '$20 a month' is the correct option.
Step-by-step explanation:
The slope-intercept form of the line equation
where
- m is the rate of change or slope
- b is the y-intercept
Given that Marta has opened a saving account with $25.
In other words, the initial condition of the function is $25.
Thus, the y-intercept b = 25
Let y be the total amount
Let x be the number of months
Given that at the end of three months, she has $85 in her account.
so we get the point (x, y) = (3, 85).
Now, in order to determine the rate of change or slope, substitute x = 3, y = 85 and b = 25 in the slope-intercept form of the line equation to determine the rate of change or slope
y = mx + b
85 = m(3) + 25
85 = 3m + 25
switch sides
3m + 25 = 85
subtract 25 from both sides
3m + 25 - 25 = 85 - 25
3m = 60
divide both sides by 3
3m/3 = 60/3
m = 20
so, the rate of change is $20 a month.
Thus, Marta has been making deposits at the rate of $20 a month to her savings.
Therefore, the option '$20 a month' is the correct option.