Why did the' 90 s change to ieth?

Going to y plus ieth is one thirtieth of the ordinal number. From 20 to 90, it becomes ordinal form, and the change is the same. Twenty ~ twenty,? ninetieth

Basic order, general; One, two, three, back by yourself; Five, eight, nine, twelve; Others are followed by th; Y ends in I, followed by eth.