Draw and use a double number line diagram to model and solve the problem below.
Leah gets to paint her room since she bought a new bedspread set. Two gallons of paint covers 800 square feet. How many gallons of paint will Leah need to paint her entire room, which is 1,200 square feet?
We could draw the following double number line diagram to represent the relation between the gallons of paint and the covered square feet:
```
Gallons of paint: 0----2----4
|====|====|
Covered square feet: 0---800--1600
```
According to the problem, 2 gallons of paint covers 800 square feet. We want to know how many gallons are needed to cover 1,200 square feet. We can see that 1,200 square feet is halfway between 800 and 1600. So, on the number line, we could mark this point:
```
Gallons of paint: 0----2----4
|====|====|
Covered square feet: 0---800***1200***1600
```
Therefore, the number of gallons needed to cover 1,200 square feet would be halfway between 2 and 4 gallons. So Leah will need 3 gallons of paint to paint her entire room.