Subtraction Within 20 Games for Kids

How to Learn Subtraction Facts to 20

To learn subtraction facts to 20, build them from the facts within 10 and the addition facts your child already knows, use a strategy for the ones that cross 10, then practice a little most days until the answers come from memory. Facts like 17 − 4 follow straight from 7 − 4. The real work is the facts that cross 10, like 13 − 5 and 15 − 8, and two strategies cover them: go down to 10 first, or think addition.

Use 10: go down to 10 first

For a teen number when the ones run out, use 10 as a stop on the way down. In 13 − 5, take away 3 to land on 10, then take away the 2 that are left: 13 − 3 = 10, and 10 − 2 = 8. In the same way, 14 − 6 = 14 − 4 − 2 = 8. This works for every teen fact that crosses 10, and it uses the same pairs to 10 practiced in our making 10 games and number bonds games.

Think addition

Every subtraction fact is an addition fact turned around. For 15 − 8, ask “8 plus what makes 15?”: 8 + 7 = 15, so 15 − 8 = 7. Common Core standard 1.OA.B.4 teaches subtraction this way, as an unknown-addend problem. A child who knows the sums in our addition to 20 games already has the answers to the subtraction facts to 20.

Use doubles

Doubles are among the easiest addition facts to remember, and each one gives a subtraction fact back: 12 − 6 = 6 because 6 + 6 = 12, 14 − 7 = 7, 16 − 8 = 8 and 18 − 9 = 9. A near double is one step away: since 8 + 8 = 16, 15 − 8 is one less than 8, which is 7.

Teen numbers without crossing 10

When a teen number has enough ones, the ten stays where it is. In 17 − 4, subtract the ones, 7 − 4 = 3, and keep the ten: 13. When both numbers are in the teens, like 18 − 13, the two tens cancel and only 8 − 3 = 5 is left; in 15 − 10, the ten comes off and 5 is left. This is the first step toward 2-digit subtraction, where tens and ones are subtracted by place.

Subtracting from 20

20 is two tens. Taking away a one-digit number breaks the second ten with a pair to 10, while the first ten stays: in 20 − 6, 10 − 6 = 4, so 20 − 6 = 14. Taking away a teen number leaves only the pair to 10 from the second ten: 20 − 16 = 4.

Common mistakes

• 12 for 13 − 5: taking the smaller ones digit from the bigger one. Seeing 3 and 5 in the ones place, a child does 5 − 3 = 2 and keeps the ten to get 12. Show that 13 is one whole pile: take 3 away to reach 10, then 2 more from the ten to land on 8.
• One off when counting back. Saying “13, 12, 11, 10, 9” for 13 − 5 counts the starting number 13 as the first step and lands on 9 instead of 8. Start the count on the number below, 12, 11, 10, 9, 8, or skip the count by going down to 10 first.

In the online games on this page, the wrong answers to the facts that cross 10 include this slip, like 12 for 13 − 5, so your child learns to spot it.

For how schools teach these strategies today, see number bonds and making ten in our guide to new math, and for the research on building recall, read math fact fluency.

What Grade Learns Subtraction Within 20?

Subtraction within 20 is a 1st and 2nd grade skill in the US, usually learned at ages 6-8. Common Core standard 1.OA.C.6 asks 1st graders to add and subtract within 20, fluently within 10, using strategies such as counting back, decomposing a number leading to a ten (13 − 4 = 13 − 3 − 1 = 9) and using the relationship between addition and subtraction, and 1.OA.B.4 asks them to understand subtraction as an unknown-addend problem. Standard 2.OA.B.2 asks 2nd graders to fluently add and subtract within 20 using mental strategies. In England, Year 1 children represent and use number bonds and related subtraction facts within 20, and Year 2 children recall and use addition and subtraction facts to 20 fluently.

Age

6-8

US grade

Grades 1-2

Standard

Common Core 1.OA.C.6, 2.OA.B.2, 1.OA.B.4

England

Years 1-2