(x*100)/300=45

x is 135. But how do I calculate it correctly?

Solve for x:
x/3 = 45

Multiply both sides of x/3 = 45 by 3:
(3 x)/3 = 3×45

(3 x)/3 = 3/3×x = x:
x = 3×45

3×45  =  135:
Answer: |  x = 135

Cheers mate!  :)