10/18.38 is a ratio the same as any number over 100 is whatever the percent is. We don't know that number so we have to solve for it. So put them equal.
set up the problem:
10/18.38 = x/100
cross multiply:
100 x 10 = 1000
18.38 x (x) = 18.38x
set it up again:
18.38x=1000
divide both sides by 18.38:
18.38x/18.38=x
1000/18.38= 54.4% <==== thats your answer