Addition and subtraction to 100 — introduction

Addition and subtraction to 100 — introduction

Addition and subtraction to 100 — introduction

In 1st grade you added and subtracted within 20. You knew that 8 + 5 = 13 and 15 − 7 = 8. Now we'll go further — all the way to 100.

Why this isn't exactly the same

Numbers up to 20 needed only two things: ones and maybe one ten. Numbers up to 100 have nine tens and nine ones. You have to work with both at once — tens separately, ones separately.

A 2nd-grader at a kitchen table combining two piles of cubes and rods

What adding means

Adding means putting two piles together. Picture two piles of pebbles — one with 24, the other with 13.

Two piles of tens and ones being combined into one

When you put them together, how many do you have? 24 + 13. By the end of 2nd grade you'll know that without hesitation.

What subtracting means

Subtracting means taking some away from a pile. You had 35 sweets, you ate 12. How many are left?

A smaller part being taken away from a single pile 35 − 12. You'll know this one too.

Two kinds of problems

Addition and subtraction within 100 come in two difficulty levels:

  • Without regrouping — when the ones add up to less than ten. For example 24 + 13 (4 + 3 = 7, no problem).
  • With regrouping — when the ones spill over into a new ten. For example 27 + 18 (7 + 8 = 15, one ten has to move across).

The next articles show both kinds step by step.

What you'll learn

In this topic you'll meet:

  • Addition within 100 without regrouping.
  • Addition within 100 with regrouping — the most important new skill of 2nd grade.
  • Subtraction within 100 in both flavours.
  • Column method — a neat written way that always helps when the numbers feel big.

Summary

  • Addition and subtraction within 100 stand on tens and ones, which you already met in the hundred chart.
  • There are two kinds of problems — without regrouping (easier) and with regrouping (more interesting).
  • Any problem can also be done in columns — we'll get to that.