We'll say that months = n.
Make a set of the DVD's sold:

In month 1, Aaron sold 20 DVDs. There is no data for month 0.
There is a constant increase of 30 DVDs every month. We can make an equation out of this to fit this data set:

a represents the DVDs made.
We need to subtract 10 in this equation, as the starting point is 20, and the increase of 30 is different from the increase from n = 0 to n = 1.
We are looking for the amount of DVDs Aaron sold on the 13th month. Plug 13 into the equation:


The predicted number of DVDs Aaron will sell on the 13th month is
380.
Answer:

Step-by-step explanation:
Multiply by 2:
2n = Z(A - m)
Divide by Z:
Subtract A:

Multiply by -1:

Answer:
About 27 weeks
Step-by-step explanation:
349.99/13 = 26.9 = 27
So you have to solve for "donation" - so let's call that X. And the things we know are what the grandfather donated, as a function - meaning based upon - Theo's donation. We'll call that Y.
So Y is equal to 1/2 of $4 plus Theo's donation...... 0.5*(x+4).
So... 4.25 = 0.5(X+4), solving for X
4.25=0.5x + 2
-2 -2
2.25=0.5x - divide each side by 0.5 to get x alone.
x = 4.50
Plugging back in:
(4.50 + 4.00)*0.5 = 4.25!