Greetings..... 21-y would be your algebraic expression...
Note.....If your wondering why its not y-21 instead, read below for clarification...
Up above it stated Y is less than... What are they saying.. well, there basically saying the algebra variable ( in other words, the letter y) is LESS THAN 21.
Since its LESS than 21....221 is GREATER... So... can you subtract y from 21 if its less? NO Can you subtract 21 from Y because y is less? YES!
