RS2 Level C, Lesson 53: Subtraction Puzzle

There’s been some chatter on the forum groups about RS2 Level C, Lesson 53 and Lesson 54 and the Worksheet 25. I thought this would be a dandy topic for this week’s blog.

Here’s the worksheet in question. It’s asking the child to complete the subtraction equations using only specific numbers.

It’s a puzzle. Maybe your child is like me; I don’t like puzzles. Well, puzzles that I’m not good at, that is! Ironically, I AM good at this once I get past the self-imposed statement of “I don’t like puzzles.” I know it’s silly, but I want you to know what’s going on in my head because I’m quite sure I’m not the only one thinking like this….

So, let’s tackle this worksheet. First, to lay the groundwork, the prior ten lessons are discussing subtraction and subtraction strategies. This is the cumulation and application of all the subtraction learned previously. So, we know how to subtract and are practicing now. It’s the puzzle part that’s the challenge.

Using only the following numbers, we are to figure out the subtraction equations:

18, 20, 21, 24, 27, 30, 32, 35, 40, 45, 49, 50, 56, 60, and 64

The first four are pretty easy.

1 = 21 – 20    this one was given to us  🙂

2 = 20 – 18    or  32 – 30

3 = 21 – 18    and a lot of other possibilities

4 = 24 – 20

Of course, there are many options. The next one, 5 = __________, I had to pause a moment because I had run out of solutions using the 20s. I jump to the 30s and see 35. I can do 5 = 35 – 30. Or it could be 5 = 40 – 35.

So how am I doing this? I’m running up and down the numbers listed, looking at numbers next to each other, or a step away from each other, and subtracting to see what might fit.

I keep going.

6 = 27 – 21

7 = 27 – 20

8 = 40 – 32

9 = 49 – 40

10 = 50 – 40    or 40 – 30    or 30 – 20   or 60 – 50

Whew. That’s all we need to do for Lesson 53. That wasn’t too bad, right?

I think it’s curious to see how the thought process goes with these answers. Did any of you look at this as an addition problem?

Let’s look at 7 = __________ again. How about approaching it as 7 plus a number on the list and see if we have that answer in the number line up?

7 + 18 = 25   Nope, that answer’s not in the list.

7 + 20 = 27   Ooooh. We have that one! And I used 7 = 27 – 20 on my worksheet.

7 + 21 = 28   Nope.

7 + 24 = 31   Nope.

7 + 27 = 34   Nope.

7 + 30 = 37   Nope.

7 + 32 = 39   Nope.

7 + 35 = 42   Nope.

7 + 40 = 47   Nope.

7 + 45 = 52   Nope.

7 + 49 = 56   Yippee! Found another!! 7 = 56 – 49

7 + 50 = 57   Nope.

7 + 56 = 63   Nope. And no need to go further because the sum would be more than 64, our highest number in the list.

The next day, Lesson 54, works with subtracting two-digit numbers. Then we are to finish this worksheet, looking for solutions to our remaining subtraction equations.

As I worked through the problems, in addition to the previous methods used in the first ten problems, I decided to jump around as I searched for solutions. Here are the numbers again:

18, 20, 21, 24, 27, 30, 32, 35, 40, 45, 49, 50, 56, 60, and 64.

I started with the list of numbers, working backwards, and it sounded like this: 64 – 50 = 14. I can put that one on the worksheet. 64 – 49 = 15. 64 – 45 = 19. See how this is going? Here’s my work in process.

You know, this isn’t too tough. I can do this! So can you. And so can your child.

This worksheet helps the child connect subtraction and addition. It helps them to try new processes and procedures by combining common sense, understanding of the relationship between subtraction and addition, and  willingness to take a risk. Yes, there is some frustration, but that is part of life. Children need encouragement to overcome frustration. Frustration is part of the process of learning and understanding!