subtract 3 from both sides:

Multiply both sides by 2:

Subtract 3 from both sides:

Change sign to both sides (this implies that you should also change the inequality sign):

Answer: 31
How: If there are two negative signs like this, you would connect them together and make it into a addition sign where you then can add them.
The scale is 1:4, as the poster's dimensions are 4 times larger than the scale drawing, which is represented by the second number in the ratio. To see this, you need to see that 15 goes into 60 four times and 20 goes into 80 four times. I hope this helped you!