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N1/E3.2 Worksheet Two ”Subtraction – decomposition” You may have been taught this subtraction method at school. It involves splitting the sum into hundreds, tens and units. Have a look at this sum: 192 - 44 Before you begin, try to get a rough idea of what the answer is going to be: 192 can be rounded up to 200, & 44 can be rounded up to 50. So… 192 - 44 is approximately 200 – 50 = 150 So before we start, we know that the final answer will be about 150. (this is called an ESTIMATE. It allows us to check our final answer is reasonable.) Now do the sum - remembering to start with the units first. Finding Ten If one of the columns has a smaller number on top, the bottom number cannot be taken away e.g. in the example below, 2- 4 cannot be done! Start with the UNITS column first. 2 is smaller than 4, so we need to add ten from the 9 lots of ten. So 9 becomes 8 and 2 becomes 12 12 – 4 = 8 Put 8 in the UNITS column. Move to the TENS column. 8-4=4 Put 4 in the TENS column. Move to the HUNDREDS column. 1+0=1 Put 1 in the HUNDREDS column So 192 - 44 = 148 Checking your answer Now we must check the answer makes sense. Before we began, we ESTIMATED the answer would be about 150 Our final answer was 148 So the answer makes sense! Exercise 1) 378 - 243 2) 499 - 345 3) 764 - 406 4) 657 33 5) 6) 963 - 200 7) 244 27 8) 553 - 107 9) 144 - 115 10) 396 - 292 754 46 Borrowing because of a zero This is the same as if one of the columns has a smaller number on top, you borrow from the number to its left. 205 - 33 Start with the UNITS column first. 5–3 =2 Put 2 in the UNITS column. Move to the TENS column 0 is smaller than 5, so we must borrow from the 2 in the HUNDREDS column So 0 becomes 10 & 2 becomes 1. 10 – 5 = 5 Put 5 in the TENS column Move to the HUNDREDS column. 1+0=1 Put 1 in the HUNDREDS column So 205 - 53 = 152 Remember: If the top number is smaller, borrow from the left. If a zero appears in the top number you will probably need to borrow. Exercise 1) 602 - 363 2) 803 - 444 3) 501 - 222 4) 904 - 137 5) 6) 208 65 7) 600 - 123 8) 900 - 497 9) 409 36 10) 703 - 567 500 - 341 Addition and subtraction are opposites. You can check the answer to a subtraction sum by turning the numbers around and adding them up. Have a look below. Here you've done the sum 10 - 25 and got the answer 15. To check the answer, turn the sum around to 15 + 10 and see if you get 25. Exercise Four – Part Three a) Check ALL your answers to the questions from exercise two, we’ve done the 1st one for you! 1) 602 - 363 239 2) 803 - 444 3) 501 - 222 As we get 602 here we KNOW that our answer for the subtraction was correct! 239 + 363 602 + 444 Don’t be a lazy toad, write the rest of them out yourself!!! + 222