## Sunday, October 27, 2013

### Adding and Subtracting 10 from any Number

In the Common Core Standards for first grade there is this little standard about students being able to find 10 more or 10 less than any two-digit number.
• CCSS.Math.Content.1.NBT.C.5 Given a two-digit number, mentally find 10 more or 10 less than the number, without having to count; explain the reasoning used.
I find this standard an important one to make sure first graders can do.  It forms the foundation for important work thinking about adding and subtracting two digit numbers later in grade 1 and into second grade.  The strategies and ideas kids share when adding or subtracting two and three digit numbers start with making sure your students master this important standard.

Here is a quick and easy routine I do with mini ten frames to ensure students have this important standard down.  The ten frames I use in this activity are available as free downloads here and here.  The first link takes you to a sheet of full mini ten frames and the second to a sheet of partial mini ten frames.  I use one vertical strip from each sheet for each kid.  This gives them each a 1-9 mini ten frame and 10 full mini ten frames.

 I start by putting out one of the partial ten frames.  I ask kids how many dots they see.
 Then I place a full ten frame next to the partial one.  I ask kids how many dots they see.  Some kids will instantly know it as a teen number and others will need to count on from one of the numbers.  Kids who are very unsure might count all the dots one by one.  If they need to count each dot by ones, I know they need more practice with adding ten to a one digit number and will go there instead of going farther with this routine.
 I place another full ten frame next to the first one and ask kids how many dots they see.  Some kids will know immediately and others might need to count some or all of the collection.
 I continue placing another full ten frame next to the previous ones.  I vary my language some and use a few different phrases as I do this.  Sometimes I say "Now how many dots?"  Other times I might say "Add ten" as I put down the card or I might also say "What is ten more."  Kids need to hear the same question asked in different ways.

 Once I have gotten as high as I wanted to go with that student.  (which is often in the 100+ range) I pick up one ten frame card at a time and ask questions such as "Now how many?, What is 10 less? or Subtract 10."  By doing this, kids are getting the idea of 10 less and also the idea of counting backwards by tens starting at a non-decade number.
 Varying the start number will give your students more practice with these big ideas
• How do you work on this important standard with YOUR students?  Respond in the comments below!