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.
What adding means
Adding means putting two piles together. Picture two piles of pebbles — one with 24, the other with 13.
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?
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.
35 − 12. You'll know this one too.