Addition and Subtraction Mental addition of 2 digit

Addition and Subtraction Mental addition of 2 -digit numbers Objectives Day 1 Add pairs of 2 -digit numbers using different strategies. Day 2 Add three 2 -digit numbers using different strategies. © hamilton-trust. org. uk 1 Year 3

Addition and Subtraction Mental addition of 2 -digit numbers Starters Day 1 Add multiples of 10 (pre-requisite skills) Day 2 Add 3 or 4 numbers (pre-requisite skills) © hamilton-trust. org. uk 2 Year 3

Addition and Subtraction Mental addition of 2 -digit numbers Starter Add multiples of 10 © hamilton-trust. org. uk 3 Year 3

Addition and Subtraction Mental addition of 2 -digit numbers Starter Add 3 or 4 numbers (pre-requisite skills) © hamilton-trust. org. uk 4 Year 3

Addition and Subtraction Mental addition of 2 -digit numbers Objectives Day 1 Add pairs of 2 -digit numbers using different strategies. © hamilton-trust. org. uk 5 Year 3

Day 1: Add pairs of 2 -digit numbers using different strategies. Work out 23 + 35 then discuss in pairs how you solved it. What strategies did you use? Another way is to partition the two numbers then add the 10 s and 1 s and recombine. One way is to start with the larger number, count on 20 then 3…. … so 35 + 20 = 55 and 55 + 3 = 58. © hamilton-trust. org. uk … so 20 + 30 = 50 and 3 + 5 = 8. Then 50 + 8 = 58 6 Year 3

Day 1: Add pairs of 2 -digit numbers using different strategies. For adding a near Let’swe trycan use multiple 47 +Fly 21 on a 100 Spider and square. . . Remember Spider counts Spider starts at 47. on or back in 10 s… He jumps down 2 squares (two tens) to 67. …and Fly counts on 21 is 1 more than 20 or back in 1 s. so count on 1 with fly. So, 47 + 21 = 68 © hamilton-trust. org. uk 7 Year 3

Day 1: Add pairs of 2 -digit numbers using different strategies. Let’s try 65 + 19 He jumps down 2 squares to 85. Spider starts at 65. This time fly goes back 1 since 19 is 1 less than 20. What will fly do this time? So 65 + 19 = 84. © hamilton-trust. org. uk 8 Year 3

Day 1: Add pairs of 2 -digit numbers using different strategies. Let’s check some of those on the 100 square or flipchart. Work with a partner, finding other pairs of 2 -digit numbers to add. Can you find a pair for each strategy? Let’s make a list on our class display of some of our strategies, e. g. partition look for number bonds look for near multiples use Spider and Fly put the larger number first. © hamilton-trust. org. uk 9 Year 3

Investigation: Adult Sheet © hamilton-trust. org. uk 10 Year 1

Investigation: Child Sheet © hamilton-trust. org. uk 11 Year 1

© hamilton-trust. org. uk 12 Year 3

Addition and Subtraction Mental addition of 2 -digit numbers Objectives Day 2 Add three 2 -digit numbers using different strategies. © hamilton-trust. org. uk 13 Year 3

Day 2: Add three 2 -digit numbers using different strategies. Who can remember some of our strategies for adding two 2 -digit numbers? Look for number bonds. Partition into 10 s and 1 s. Look for near multiples. Use Spider and Fly. Put the larger number first. © hamilton-trust. org. uk Today we will use all these to find the most efficient ways to add three 2 -digit numbers. 14 Year 3

Day 2: Add three 2 -digit numbers using different strategies. 18 cm 43 cm 25 cm How could we find the total length of these 3 ribbons? 40 + 20 + 10 = ? Let’s try partitioning into 10 s and 1 s and putting the larger numbers first. 8+5+3=? 70 + 16 = ? © hamilton-trust. org. uk 15 Year 3

Day 2: Add three 2 -digit numbers using different strategies. 23 cm 57 cm 48 cm What about these three ribbons? 50 + 40 + 20 = ? Let’s try partitioning into 10 s and 1 s. Did you spot the pair of 1 s that make 10? 8+7+3=? 110 + 18 = ? © hamilton-trust. org. uk 16 Year 3

Day 2: Add three 2 -digit numbers using different strategies. 65 27 35 31 48 Are there three of these numbers that would be straightforward to add? Which group of three could be trickiest? 65 , 35 and 21. 5 + 5 = 10 to help us add the 1 s. 60 + 40 + 20 = ? 48, 65 and 27 could be trickier… Let’s try partitioning. 8+7+5=? 120 + 20 = ? © hamilton-trust. org. uk 17 Year 3

Challenge © hamilton-trust. org. uk 18 Year 3

Addition and Subtraction Mental addition of 2 -digit numbers Well Done! You’ve completed this unit. Objectives Day 1 Add pairs of 2 -digit numbers using different strategies. Day 2 Add three 2 -digit numbers using different strategies. © hamilton-trust. org. uk 19 Year 3

Problem solving and reasoning questions Use a different strategy for each of these additions: (a) 45 + 29 (b) 45 + 34 (c) 65 + 35 (d) 78 + 28 Explain why you chose a particular strategy for (a) and (c) Complete the bar model diagrams: Choose three of these numbers and add them together. 28 67 39 14 82 53 Which three have a total closest to 200? © hamilton-trust. org. uk 20 Year 3

Problem solving and reasoning questions answers Use a different strategy for each of these additions: (a) 45 + 29 74 Add 30 and subtract 1 (add and adjust). (b) 45 + 34 79 Add 30 then 4 (or partition and add). (c) 65 + 35 100 Complements to 10 or 100. (d) 78 + 28 106 Partition, add and recombine. Explain why you chose a particular strategy for (a) and (c) Children may use other strategies but are they the most efficient? The key thing is to look at the numbers and see what they suggest might be the most efficient. Complete the bar model diagrams: Choose three of these numbers and add them together. 28 67 51 14 82 53 Which three have a total closest to 200? 67 + 53 + 82 = 202 © hamilton-trust. org. uk 21 Year 3