Students will learn how to find 1 more, 1 less, 10 more, or 10 less. To find 10 more or 10 less, the units stay the same, only the tens place changes. Students should not count to find 10 more, or 10 less. For example, to find 10 less than 46, students should not count 46, 45, ..., 36.
Adding 1 vs. adding 10 by Khan Academy
Add 1s or 10s (no regrouping) by Khan Academy
Next, give your students this challenge:
2018 Math Kangaroo Levels 1-2 Problem #18 by STEM4all
Conclude by leading this investigation:
1.NBT.C.5: Given a two-digit number, mentally find 10 more or 10 less than the number, without having to count; explain the reasoning used.