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Equal Additions I am learning to solve subtraction problems by equal additions that turn one of the numbers into a tidy number. Numbers like 10, 20, 30, 40 etc. Alice has $304 and spends $198. How much money does she have left? $304 - $198 Step 1: We are trying to find the difference between these two numbers so let’s imagine them on a number line. Step 2: If we move $198 up to $200 this question would be much easier to answer. Because we are moving one end up by 2 we must move the other end up by 2 to keep the distance between the two numbers the same. Note how the distance between the two Step 3: 306 – 200 = 106 numbers hasn’t changed so the answer will be the same. $200 $198 +2 $304 $306 +2 Below is the same problem but this time I have shown how I like to record my working. You might like to do this too until you are ready to do the whole process in your head. Let’s see what happens if I change the first number into a tidy number. $304 - $198 = …. $304 - $198 = …. +2 +2 -4 -4 $306 - $200 = 106 …. $300 - $194 = 106 …. I chose to increase $198 to $200 to make this question easier to answer. Both sides are getting bigger by 2. I do the same to both sides so I can keep the gap the same. Since I subtracted 4 from one side I had to subtract 4 from the other side. Notice how this question gets the same answer but it is harder to work out. Always change the second number. Not the first Let’s try this example together. Try to do each step in your head before clicking. 724 – 299 = ? Step 1: Which number will you change and what tidy number will you change it into? 724 - 299 = …. +1 +1 …… 725 - …… 300 = 106 …. Step 2: You have changed 299 into a tidy number by adding 1. What do you need to do to the other number? Step 3: What is the answer? Now try these questions on your own. Remember to first change the question then answer. 1) 237 – 198 = ? 239 - 200 = 39 2) 435 – 297 = ? 438 + 300 = 138 3) 684 – 499 = ? 685 + 500 = 185 4) 514 – 398 = ? 516 + 400 = 116 5) 317 – 97 = ? 320 + 100 = 220 Click again when you have completed each question and the answer will be displayed. Need some more practise? Try Pg52 of Teacher Tools Addition and Subtraction Book Numeracy resources