Home › Community › Questions & Answers › Mental Math Q & A › How to use L —> R method in subtraction 590492
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August 13, 2018 at 6:32 am #5026
As 54 is 1, 99 is 0 now how to convert 54 = 1 in 0 for last unit digit 02 and ten digit 99 = 0 in 9.
August 13, 2018 at 6:35 am #5028There are two ways to approach this problem. The normal LR method which is a bit tedious when you have to borrow numbers and the rounding up LR method which is more efficient to use when you have to borrow numbers.
In this problem you have to borrow numbers.
Normal LR Method
Here are the steps for the LR method without rounding up.
1. Subtracting the first two digits (5 – 4) we get 1.
590 – 492 = 1 _ _
2. Subtracting the second two digits (9 – 9) we get 0.
590 – 492 = 1 0 _
3. We can’t subtract the last digits 0 and 2. So we borrow 1 from our answer 10, so the calculated digits change to 9.
590 – 492 = 9 _
4. Next we subtract 10 – 2 to get 8.
590 – 492 = 9 8
LR Method With Rounding Up
As mentioned in the video lessons, you will be able to simplify this problem by rounding up the second number. Rounding up is recommended when you have to borrow numbers during subtraction.
1. So 492 can be rounded up by 8 to get 500. So if we write 492 as (500 – 8) the problem becomes:
590 – (500 – 8) = _ _
2. Subtracting 590 – 500 we get 90.
590 – 500 = 90
3. Adding the amount we rounded up 8 to 90 we get 98.
90 + 8 = 98.
There are only 3 steps to do this subtraction if you round up.
Hope this clarified. Let me know if you have any further questions.
August 13, 2018 at 6:37 am #5030Here my problem is same, via normal LR method….
1. 590 – 492 = 1_ _
2. In ten digit 90 – 92 = 0_
3. Now after borrow( for unit digit) 0 become 9(in ten digit) and unit digit is 102 = 8
4. But how I change hundred digit 5 – 4 in zero.
chk ur solution, u didn’t mention how u make 1 to 0, u just make 99 = 0 in 9 after carry 1,
<figure>2. Subtracting the second two digits (9 – 9) we get 0.590 – 492 = 1 0 _
3. We can’t subtract the last digits 0 and 2. So we borrow 1 from our answer 10, so the calculated digits change to 9.
590 – 492 = 9 _
4. Next we subtract 10 – 2 to get 8.
590 – 492 = 9 8</figure>
August 13, 2018 at 6:39 am #5032Hi Rupesh,
The steps I mentioned in my first reply are correct. The second step in your last reply is incorrect:
2. In ten digit 90  92 = 0_
. The correct second step is 9 – 9 = 0. In the third step I am borrowing 1 from the answer 10 to get 9.Here are the steps from my original reply again with additional comments.
In 590 – 492:
1. Subtracting the first two digits (5 – 4) we get 1.
590 – 492 = 1 _ _
2. Subtracting the second two digits (9 – 9) we get 0. [This is the step you are doing incorrectly. You should subtract one digit at a time. The intermediate answer after step 2 is now 10 not 1. I think you are subtracting 90 – 92 instead of subtracting 9 – 9.]
590 – 492 = 1 0 _
3. We can’t subtract the last digits 0 and 2. So we borrow 1 from our answer 10. [Note: Our answer is not 1. It is 10. I think you are borrowing one from 1 instead of borrowing one from 10.] If you borrow one from 10, the calculated digits become 9.
590 – 492 = 9 _ (Got by borrowing 1 from 10 which is 10 – 1 = 9)
4. Since we borrowed 1 in the last step, we attach the borrowed 1 to 0 to get 10. Then we can subtract 10 – 2 to get 8.
590 – 492 = 9 8 (Got by attaching the borrowed 1 to 0 and subtracting 10 – 2)
You are trying to do the problem by subtracting 90 – 92 in the second step instead of subtracting one digit at a time which is 9 – 9. If you do 90 – 92 in the second step then there will only be 3 steps in the calculation instead of 4 and the third step of the calculation will be more complicated. Here are the steps if you do 90 – 92 in the second step:
1. Subtracting the first two digits (5 – 4) we get 1.
590 – 492 = 1 _ _
2. To subtract the next two digits (90 – 92) we need to borrow 1 from the answer 1 so it becomes 0.
590 – 492 = 0 _ _
3. Because we borrowed 1 in our previous step, we attach one to 90 to get 190. Subtracting 190 – 92 we get the answer 98.
590 – 492 = 0 9 8 (Got by attaching the borrowed 1 to 90 and subtracting 190 – 92)
You are subtracting 90 – 92 instead of 9 – 9. If you subtract 90 – 92, after borrowing 1 you must now subtract 190 – 92 to get the correct answer 98. Subtracting one digit at a time (i.e. 9 – 9) instead of subtracting two digits at a time (i.e. 90 – 92) simplifies the problem.
Let me know if this clarifies.
Warm Regards,
Abhishek

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