You would write it like this 21,000,000 or 2.1*10^6.
Another Answer:-
It is the same as in scientific notation and it is: 2.1*10^7
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You would write it like this 21 or 2.1*10^1.
20,000,000,000,000,000,000,000
21,000,000
It is already in standard form.
In its simplest form 525/1000 = 21/40