#### Answer

60%

#### Work Step by Step

Step 1: We make the assumption that 90 is 100% since it is our output value.
Step 2: We next represent the value we seek with $x$.
Step 3: From step 1, it follows that $100\%=90$.
Step 4: In the same vein, $x\%=54$.
Step 5: This gives us a pair of simple equations:
$100\%=90(1)$.
$x\%=54(2)$.
Step 6: By simply dividing equation 1 by equation 2 and taking note of the fact that both the LHS
(left hand side) of both equations have the same unit (%); we have
$\frac{100\%}{x\%}=\frac{90}{54}$
Step 7: Taking the inverse (or reciprocal) of both sides yields
$\frac{x\%}{100\%}=\frac{54}{90}$
$\Rightarrow x=60\%$
Therefore, $54$ is $60\%$ of $90$.