A second, quick revision of counting on. Whilst some children may well still be starting with the given number and then counting on with their fingers others will be using more sophisticated methods such as:
to count on 9, add 10 and subtract 1
to count on 11, add 10 and add 1
to count on 8, add 10 and subtract 2
to count on 12, add 10 and add 2