2.NBT.B.8: Add or take 10 or 100 from a number between 100 and 900 in your head

Mentally add 10 or 100 to a given number 100–900, and mentally subtract 10 or 100 from a given number 100–900.Common Core State Standards for Mathematics, as adopted in the Massachusetts Curriculum Framework for Mathematics (2017), Grade 2.

Play the game: Count On, Count BackStep 29 of the ColorMyMath curriculum · free, no sign-up →

How ColorMyMath teaches it

Past nine, color tells you which place a digit lives in. A ten is a triangle of ten dots; the number of triangles is the tens digit, so 34 is three triangles and four loose green dots.

Every page is free and open — nothing to install, no account, and it works on a phone or a classroom board. Every number has its own color: 1 red, 2 orange, 3 yellow, 4 green, 5 blue, 6 purple, 7 brown, 8 pink, 9 cyan, and a number is that many colored dots.

Where it sits in the curriculum

• Step 29Adding 10: counting to 150 by 10s
• Step 57Complements of 100: 50+50, 60+40, 70+30, 80+20, 90+10

More Grade 2 standards in Number and Operations in Base Ten

• 2.NBT.A.1Hundreds, tens and ones in a three-digit number
• 2.NBT.A.2Count within 1000; skip-count by 5s, 10s and 100s
• 2.NBT.A.3Read and write three-digit numbers: numerals, words, expanded form
• 2.NBT.A.4Compare three-digit numbers with >, = and <
• 2.NBT.B.5Add and subtract within 100 fluently using place value
• 2.NBT.B.6Add up to four two-digit numbers: gather the tens first, then the ones
• 2.NBT.B.7Add and subtract within 1000; sometimes tens or hundreds are made or broken
• 2.NBT.B.9Explain why addition and subtraction strategies work

ColorMyMath · getridoftheteacher.com · every number has its own color, and a number is that many colored dots. Matching a game to a standard is a judgement call; the official text is at doe.mass.edu.