To find x in the equation 18:5=72:x, we can set up a proportion:

18/5 = 72/x

Cross multiply to solve for x:

18x = 360

x = 360/18
x = 20

Next, to find x in the equation ×:9=35:15, we can set up a proportion:

x/9 = 35/15

Cross multiply to solve for x:

15x = 315

x = 315/15
x = 21

Finally, to find x in the equation 55:x=5:3, we can set up a proportion:

55/x = 5/3

Cross multiply to solve for x:

3 * 55 = 5x
165 = 5x

x = 165/5
x = 33

Therefore, the value of x in the equation 55:x=5:3 is 33.