I assume the “!0” was a typo and you meant to say 10.
The nice thing about 10s is that you can treat them like 1s and add the zeros later.
So if we remove the 10s and just count them as 1s we get 7. Then just add the zero to make it 70. We can do a similar thing with the 20 by pretending it is a 2 and adding the zero later. (Pretend 70 is 7 once-more.) and just add 7 to 2 which is 9 then add the zero to make it 90.