She can Hang up 4 posters. Because Taylor only has 16 tacks.
16/4=4
Dimension of smaller garden :
l = 1/2 ft.
b = 2/3 ft.
Dimension of bigger garden :
L = 4 ft.
Let , breadth be x ft.
We know , area is given by :
Area = L×B.
Area of small garden = 
Area of big garden 
Hence, this is the required solution.
For the sake of it c=x
13.50+9x=36
36-13.50=22.5
9x=22.5
22.5/9=2.5
X=2.5