a. using Excel we get:
the equation of best fit is:
y = 12.165x + 31.335
where y represents the account balance and x the days.
b. The slope represents how much the balance increase from one day to the next one. The y-intercept represents the balance at day zero, that is before they started the store.
c. Their goal is to reach $900. Replacing y = 900 into the equation:
900 = 12.165x + 31.335
900 - 31.335 = 12.165x
868.665/12.165 = x
71.4 days = x
That is, they will need 72 days to reach their goal.
d. using Excel we get:
The difference between this correlation and the previous one is caused by the inclusion of new data. After the addition of new points is hardly impossible that the new line remains equal to the previous one.
with this new correlation, they will reach their goal after:
900 = 10.333x + 36.433
900 - 36.433 = 10.333x
863.567/10.333 = x
83.6 days = x
That is, they will need 84 days to reach their goal.
e. The second prediction is more reliable, because the line of best fit was gotten using more points.