s = # of soft drinks
b = # of burgers
d = # of dogs
[tex]s + b + d = 720 \\ 2d = b \\ 3s = b \\ \\ 2d/2 = 3s/2 \\ d = \frac{3}{2}s \\ \\ s + 3s + \frac{3}{2} s = 720 \\ 5.5s = 720 \\ 5.5s/5.5 = 720/5.5 \\ s = 130[/tex]
130 soft drinks were sold that night.
Hope this helps!