Answer:
There are a few ways:
You can do counting like this...
1000
999
998
997
996
995
994
993
Another better and faster way will be to do 3+990
The reason is becuase 7-10=3 and also that we can remove the 10 from 1000
So those were 2 ways how she can subtract CORRECTLY!
Have a great day!
:D