Simplify the following text for a Grade 4 native English-speaking person.
Original: Each unit ( 1 , 2 , ... , 9 ) is assigned a separate letter , each tens ( 10 , 20 , ... , 90 ) a separate letter , and the first four hundreds ( 100 , 200 , 300 , 400 ) a separate letter .
Simplified: Each unit ( 1 , 2 , ... , 9 ) is assigned a separate letter , each tens ( 10 , 20 , ... , 90 ) a separate letter , and each hundreds ( 100 , 200 , ... , 900 ) a separate letter .