Question

4 days I have 140 copies of a book, 9 days I only have 50 left. Write an equation where c(t)=my+b where c is number of copies still on hand and t days being available.

Answer 1

`c(t)=-18t+212`.

Explanation:

We have been given that 4 days I have 140 copies of a book, 9 days I only have 50 left. We are asked to write an equation
`c(t)=my+b`, where c is number of copies still on hand and t days being available.

We have two points on line (4,140) and (9,50). Now, we will use these points to find slope as:

`m=(140-50)/(4-9)`

`m=(90)/(-5)`

`m=-18`

Now, we will use point-slope form of equation
`y-y_1=m(x-x_1)` and substitute
`m=-18` and coordinates of point (9,50) as:

`y-50=-18(x-9)`

`y-50=-18x+162`

`y-50+50=-18x+162+50`

`y=-18x+212`

Therefore, our required equation would be
`c(t)=-18t+212`.