Comments on classification activity This page briefly explains the mental methods used by the different children and classifies the methods using the scheme discussed in 'Mental methods for subtraction'. Some methods may display characteristics of more than one of the classifications. Amy Amy chooses complementary addition to compute 74 -38. She adds 2, 30 and then 4 to 38 to build up to 74. She knows she has added 36 in all and that's her answer. Back to Classification activity Ann Ann has first taken away 20 from 83 to obtain 63 and then taken away another 7 to get the answer of 56. Thus she has used subtraction in stages for this question. Back to Classification activity Ben Ben starts with the subtrahend 99 and adds first a 1 and then 23 and 500 to build up to the minuend. This method can therefore be classified as complementary addition. Back to Classification activity Fikeni Fikeni seems to feel more comfortable with subtraction when the number has a 0 at the end. He rounds up 74 to the next ten and thinks of it as 80-6. He then takes away 38 from 80 to get 42 and then takes away the extra 6 he has added on for rounding. So his approach can be categorised as rounding. Back to Classification activity Jim Jim subtracts from left to right. He first takes away 20 from 80 to obtain 60. He then takes away 7 from 3 to get -4. 60 and -4 then give the answer 56. This inventive method can be placed in the category of other methods. However the fact that Jim starts with 80 and 20 may well justify placing this in the method of rounding. Back to Classification activity Loretta Loretta has used complementary addition to carry out the subtraction. She starts by adding a 3 to 27 to make 30 then adds 50 to get 80 and finally another 3 to make up 83. Back to Classification activity Mary Given 83 - 27, Mary first rounds 83 down to the nearest ten and subtracts 27 from 80. To the resulting 53 she adds the 3 back on and gives the answer as 56. This is another example of rounding. Back to Classification activity Nina Nina took away 40 instead of 38, so this is an example of rounding. However, as she was thinking of 38 as 40 - 2 and first she dealt with the 40 and then with the 2, you could also argue that it is an example of subtraction in stages. Students sometimes use a combination of methods depending on the algorithm. Back to Classification activity Peter Peter like many children prefers to deal with round numbers. He therefore thinks of 378 as 400 - 22. He takes away 400 from 546 and then adds on the 22 as he knows he has taken away 22 too many. He too has used rounding for his subtraction. Back to Classification activity Sam Sam has used complementary addition for his subtraction. He starts with 378 and adds a 2 to make it 380. Then he adds 20 , 100 and finally 46 to obtain 546. In all he has added 168 which is his answer. Back to Classification activity Sunil This is another example of subtraction in stages. Sunil finds it is easier to take away 34 from 74 to get 40 and then take away the remaining 4 to get the final answer of 36. Back to Classification activity Tess For this question, Tess avoids two regroupings that are necessary by the classic algorithm and takes an sensible way out. Noticing that 98 is quite close to 103, she simply counts up from 98 to 103 and gives the answer 5. This is another example of complementary addition. Back to Classification activity Back to 'Top' of Classification activity