10 more adds 1 to the tens digit. 10 less takes 1 from the tens digit.
Worked example
What is 10 more than 80?
Answer:90
10 more changes the tens digit by 1 and keeps the ones. 80 → 90. Answer: 90.
For grown-ups: 10 more / 10 less mentally. Below: three problems to try with the worked answers, and a free printable sheet. Children can practise the same skill as a game, with no sign-up and no ads.
CCSS 1.NBT.C.5
What this practises, for teachers
Practices 10 more / 10 less mentally (1.NBT.C.5). A printable sheet with its answer key, worked answers on the page, no accounts.