You just need to write a proportion between the correspondent sides:
[tex]45\div 5=18\div A[/tex]
Now, since 45 is 9 times 5, 18 must be 9 times A, and thus A must be 2.
Even if we didn't realize that, we can solve for A the usual way:
[tex]45A=5\cdot 18 \iff A=\dfrac{5\cdot 18}{45}=\dfrac{90}{45}=2[/tex]
