Here are two cards. Choose a card to make each calculation correct. One is done for you. And then we have four different calculations: four plus one equals five, 23 what one equals 22, 40 what one equals 39, and 19 what one equals 20.
Look at the two cards that we’ve been given to help us answer this problem. They’ve both got symbols on them. The first symbol means add or plus and the second symbol means take away or subtract. Each of these four calculations needs an add or a takeaway sign in it. We can see that the first calculation has been done for us. Four add one equals five. How do we know which symbol to use? Let’s think about what happens to a number if we add to it or if we take away from it.
If we start with a number and then we add to it, the answer or the total will be larger than the number we started with. The number becomes bigger. When we take away from a number, the number that we’re starting with becomes smaller. And so, the new number is less than it was to start with. So we can answer this question without working out the calculations. We just need to look at how the numbers change.
The first calculation we need to work out begins with 23. The answer is 22. The number has become smaller. Have we added or taken away? 23 take away one equals 22. The next calculation begins with the number 40 and the answer is 39. Has the starting number become bigger or smaller? 39 is smaller than 40. We must have taken a number away. 40 take away one equals 39. Our final calculation begins with the number 19 and has an answer of 20. Is the answer greater or smaller? 20 is greater than 19. We must have added to 19 to get the answer 20. 19 add one equals 20.
To find each symbol, we didn’t work out each calculation. Instead, we looked at how the number has changed. If the starting number becomes greater, we’ve added to it and we need to use the add symbol. If the starting number becomes smaller, we’ve taken away from it and we need to use the take away symbol. 23 take away one equals 22. 40 take away one equals 39. And 19 add one equals 20. The missing symbols from top to bottom are take away, take away, add.