Addition and subtraction within 100

Addition within 100 in 2nd grade

Addition within 100 is the big step of 2nd grade: from facts like 6 + 7 to two-digit problems like 46 + 27 and 82 − 35. The Common Core asks 2nd graders to add and subtract within 100 fluently using place value, properties of operations and the relationship between addition and subtraction. In England the same work spans Years 2 and 3. These adding to 100 games start with tens (40 + 30), then two-digit plus one-digit, then two two-digit numbers.

Mental strategies before the written method

Before children write problems in columns, they learn to think in tens and ones. 46 + 27: add the tens (46 + 20 = 66), then the ones (66 + 7 = 73). Or split both: 40 + 20 = 60, 6 + 7 = 13, 60 + 13 = 73. Subtraction within 100 works the same way backwards. The hundreds chart picture makes it visible with 42 + 27: one row down is ten more. Children who can do this in their heads understand the column method much better when it comes.

When the ones go over ten

In 46 + 27 the ones make 13, so a new ten appears. That is regrouping, and it has its own page with a step-by-step example. This page mixes problems with and without regrouping; the settings let you choose. Start with two-digit addition and subtraction without regrouping, like 42 + 27 or 69 − 27, and add the regrouping problems when those are easy.

Games and worksheets within 100

The trainer uses a keypad, because the answers are too big to guess from four buttons. The gates runner shows "+20" or "+7" gates, so the crowd grows in tens and ones. The speed test works best with the "tens only" setting at first. Print the tens sheet, the mixed sheet with an answer key, and the blank hundreds chart for practice with jumps.

A tip for parents

Use money: in the US, dimes and pennies are tens and ones, so 46 cents is 4 dimes and 6 pennies. Adding 27 cents means 2 more dimes and 7 more pennies - and when there are 13 pennies, ten of them become one dime.