Home › Community › Questions & Answers › Mental Math Q & A › When should I round up for subtraction or addition?

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- January 27, 2018 at 9:42 am #5090
I understand how to add, subtract and round up. The only thing I do not understand is when should I round up when adding and subtracting. Should I always be rounding up before adding and subracting or is there some sort of way to know whether it is needed or not? I sometimes feel that rounding up and then later adding or subtracting that number creates another unnecessary step. But in other cases I feel that it could of been used for that task… The problem is, is that it takes me too much time for me to contemplate whether or not I should round up.

Any advice or clarification on the matter?

January 27, 2018 at 9:43 am #50923Hey Patrik,

This is a great question. The short answer is you will benefit from rounding up when you have to carry over (for addition) and borrow (for subtraction) a lot of numbers.

Lets take addition for example. Say you want to add 9898 + 4343. If you did the straight forward LR addition, you will have to carry over a number in almost every step. You could do this, but if you round up the first number, it will make things so much more easier. 9898 (rounded up is 10,000). 10,000 + 4343 = 14343. Then subtract the amount you rounded up which is 102 to get 14241. Try to do that in your head, with and without rounding up and you will realize you are flexing fewer neurons when you round up.

Now do you round up for every calculation? You don’t. Lets say you want to add 4343 + 1234. You don’t have to carry over any number to do this calculation so the straight forward LR addition makes sense. But for the fun of it, try to do the same problem by rounding up. You might find that rounding up only increases the mental effort required to do the math.

Okay we talked about addition, so what about subtraction? Rounding up is more valuable for subtraction than it is for addition. This is because borrowing numbers in your head is a lot harder to do compared to carrying over numbers. In subtraction you round up the number when you have to borrow a lot of numbers. Example 53,441 – 49,898. In this subtraction, except for the first numbers (5 – 4) you have to borrow a number during every step. So it makes sense to round up the second number before doing the subtraction. Rounding up we get 50,000 and we rounded up by 102. Subtracting 53,441 – 50,000 we get 3441. Adding 102 we get 3,543. Now try doing the same problem without rounding up. It might seem more strenuous to do it without round up.

Do you round up before doing every subtraction? Not really. Subtract 9889 – 4343. You can do the straight forward LR subtraction here to get the answer. You can do rounding up and solve the math but it will only result in an unnecessary step.

Let me know if this clarifies.

January 27, 2018 at 9:45 am #5094Wow! Thank you for the answer you have given me. I understand now through the examples and your answer clarifies 100% of my questions. I can’t thank you enough for a fast and detailed answer that leaves me 100% sure of what I should do when adding and subtracting and especially what kind of strategy and mindset I have to have prior and during calculating numbers!

Thanks so much! What an amazing course!

January 27, 2018 at 9:47 am #5096Always happy to help Patrik. If you enjoyed the course so far, please dont forget to leave a review for other future students. It really helps a lot.

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