The house has 28 windows, and their sizes are 2, 3, 4, or 5 feet.
The number of windows is proportional to their sizes.
The boy can escape through windows of size at least 4 feet.

Let the number of windows of size 2, 3, 4, and 5 feet be in the ratio 2:3:4:5.

Let the constant of proportionality be

Number of 2-foot windows =2x
Number of 3-foot windows =3x
Number of 4-foot windows =4x
Number of 5-foot windows =5x
The total number of windows is 28 

2x+3x+4x+5x=28
14x=28
x=2

By putting value

Number of 2-foot windows =2x=4
Number of 3-foot windows =3x=6
Number of 4-foot windows =4x=8
Number of 5-foot windows =5x=10

The boy can escape through the 4-foot and 5-foot windows, so the total number of windows available for escape is:
   8+10=18