Adding two 2-digit numbers

The prerequisites for this activity were to be able to add together multiples of 10 in their head and know how to count on using a 100 square.

I decided to focus on mental methods for adding 2-digit numbers as I feel it's important they understand the basics before moving on to the quicker (and arguably easier) standard written methods.

The method I taught them to use is pictured above, which I went through on the whiteboard using coloured pens for simplicity. First, I wrote out an example question with the Tens in red, the Units in purple and the equation signs (and subsequent answer) in blue: 14 + 48 =

I told them the first step was to look at the Tens. What are the Tens? 10 and 40. So what is 10 + 40? 50. I wrote this sum underneath in red, to keep the colour co-ordination.

Next, look at the Units. What are they? 4 and 8. Which Unit is bigger? The 8.

So keep the Tens total (50) and add on the bigger Unit (8) to get 58. I wrote this underneath, colour co-ordinated.

How many Units are left to add on? 4. So I wrote 58 + 4 =

They could then use their 100 squares to find 58 and count up 4 more squares to calculate the total. 62. So I wrote 62 as the answer to both 58 + 4 and the overall question of 14 + 48.

Reading through, this looks like it took a long time to explain! But it really didn't, and with a few more examples and me reminding them of the steps the girls soon got the hang of this method. This is my personal method of choice for mental addition, which is the reason I taught it to them. As they get older and their mathematical understanding strengthens, insha'Allah, then I'll introduce the alternative methods for them to choose from. 😊

After two examples using numbers of my choice (one where the Units total to within 10, another where they didn't), I asked the twins to choose numbers at random for me to use in the questions - otherwise I wouldn't have used 49 + 94 as an example since it goes into the Hundreds! But Alhamdulillah, this was the last example we did together and they understood it fine. 👍

The next day - while leaving the examples on the whiteboard for reference - I gave them the pictured double-sided worksheet to do (taken from Schofield&Sims - KS1 Problem Solving 2). We did the first side (5 questions) together, with me scaffolding their thinking by reminding them of the steps to the method, and I helped with the first couple on the next page too. Then I let them try the last few by themselves and they were both able to do the final two questions completely independently, MashaAllah. 💕