Order the numbers from least to greatest.
0.21,1/5 , 0.18, 24%

When ordering numbers, it's helpful to have them all the same "type"; that is, all fractions, all percentages, or all decimals.  This means we need to convert 1/5 and 24% to decimal form.

1/5=0.2
24%=0.24

So our new sequence of numbers is {0.21,0.2,0.18,0.24}.  Ordering them from least to greatest, we get:

{0.18,0.2,0.21,0.24}
 
Converting the appropriate numbers back to their original form:

{0.18,1/5,0.21,24%}

So that's your answer.