10 More and 10 Less

I have students use base ten blocks to model 35 and 36.  Guide the students to think about their numbers:

• Which number is less, 35 or 36?  (36)
• How does your model show that 35 is less than 36? (35 has fewer ones blocks than 36)
• What number is one more than 36?  How do you know? (37 is one more, it comes just after 36 when I am counting.)

Read the following problem to the class: Tony has 22 markers.  Pat has 10 less markers than Tony.  Jan has 10 more markers than Tony.  How many markers does each child have?

• How can you use a model to find the number of markers that Pat has? (Pat has 10 fewer than Tony, so I start by showing Tony’s markers.  He has 22, 2 tens and 2 ones.  If I take 1 ten away, that leaves 1 ten and 2 ones, or 12.)
• How can we use a model to find how many markers that Jan has?  (Jan has 10 more than Tony.  Tony has 2 tens, and 2 ones.  If I add 1 ten, then Jan has 3 tens and 2 ones, or 32 markers)

I have students draw a picture to show how many markers each child has.  Point out that there is a quicker way to solve our problem rather than writing or drawing it.

• When we find 10 more or 10 less, what happens to the number of ones? (They stay the same.)

Explain that because the tens digit only changes by 1, we can find 10 more or 10 less by mentally adding or subtracting 1 to the number of tens.

I model this by selecting a number and asking students to determine what number is 10 more and what number is 10 less.  For example – I write the number 33 on the board, and draw the tens and ones.  Draw the number that is 10 more and 10 less.

• What do the numbers on the left show? (2 tens 3 ones, 23)
• How does it compare to the model for 33? (It has one less ten.)
• How do the numbers show that 43 is 10 more than 33? (The tens place changes from 3 to 4.  That means there is 1 more than in 43 than there is in 33.)
• What number is 10 more than 43? (53, the tens place changes from 4 to 5.)

For the independent practice portion, I give each student 2 number cubes (dice).  They roll the dice and record the two digit number.  Then then draw the number using tens and ones, and determine the number that is 10 more and 10 less.

For those struggling students, I pull them into a small group and we use a hundred chart and practice counting by tens.  

I also give them base ten blocks to help with building their numbers on the independent activity..  I write a number on a white board and ask students to model the number.  For example:

• How many tens are there in 47? (4 tens)
• How many ones are there in 47? (7 ones)

Then have students add one more ten to their model and ask “What number is 10 more than 47?” Have students go back to the original number (47) and take away one ten, ask “What number is 10 less than 47?”