I think one of these should be it
9,900 is 900+9000
as far as what place 100th place and 1000th place.
as far as amount of zeros 2 zeros and 3 zeros
as far as simplification of numbers 9*100 and 9*1000
something else I learned in middle school I think 9.0* 10^2 and 9.0*10^3