Students will learn how to add within 100, with and without regrouping, using base ten blocks, place value disks, and number lines. Make sure to include problems where exchanging for a ten is necessary. Students will also learn how to add multiples of ten using a hundreds chart. For example, 20 + 30 = ? Here's a lesson on that.
Addition and subtraction with number lines by Khan Academy
Add within 100 using a number line by Khan Academy
Next, give your students this challenge:
2018 Math Kangaroo Levels 1-2 Problem #17 by STEM4all
Conclude by leading this investigation:
1.NBT.C.4: Add within 100, including adding a two-digit number and a one-digit number, and adding a two-digit number and a multiple of 10, using concrete models or drawings and strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; relate the strategy to a written method and explain the reasoning used. Understand that in adding two-digit numbers, one adds tens and tens, ones and ones; and sometimes it is necessary to compose a ten.