For English speaking people, the numbers are in alphabetical order.
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it is 7
They are arrange alphabetically.
-1,-2,-3 10-1=9 9-2=7 7-3=4
This sequence of numbers is in alphabetical number when written in their full form, eg; "eight".
I'm guessing your sequence is 1, 3, 6, 10, 15, ... In which case it continues: 21, 28, 36, 45, 55, 66, ... (These are the triangular numbers.)