How to add two-digit numbers that don't end with a zero
To add any two-digit numbers that don't end in zero round the numbers up or down to the nearest multiple of 10.
EXAMPLES
17 + 26 -> Round the 17 up to 20 and the 26 up to 30 so now we're adding 20 and 30.
We now have: 20 +30 = 50 (We'll refer to this as the intermediate answer)
We added 3 to 17 to get 20 and 4 to get 26 for a total of 7. We must subtract this from the intermediate
answer to get the final answer: 50 - 7 = 43
My Little Rule: Whatever you do at the beginning you must do the equal and opposite at the end. In other
words if add x amount at the beginning when rounding up, subtract that same amount at
the end to get the final answer.
44 + 37 -> Round the 44 up to 50 and the 37 up to 40 so we're adding 50 and 40.
We now have: 50 + 40 = 90
We added 6 to 44 to get 50 and we added 3 to 37 to get 40.
Subtract this total(6 + 3 = 9) from our intermediate answer to get our final answer: 90 - 9 = 81
66 + 67 -> Round both numbers up to 70.
We now have: 70 + 70 = 140 (7 + 7 = 14 and add zero on the end)
We added 4 to 66 and 3 to 67(for a total of 7) during the rounding up process so we must subtract this
from the intermediate answer to get the final answer: 140 - 7 = 133
62 + 98 -> Round 62 down to 60 and 98 up to 100
We now have: 60 + 100 = 160
This is actually our final answer. Why? Whenever you round up and down by the same amount there
is nothing to subtract from the intermediate answer. For this example it's like we took 2 away from 62
to get 60 and gave it to 98 to get 100 then we just added them.
41+79 -> Round 41 down to 40 and 79 up to 80
We now have: 40 + 80 = 120 (8 + 4 = 12 and add zero at the end)
22 + 78 -> This time we'll round both numbers up just to show there are many different ways to solve a problem.
Round 22 up to 30 and 78 up to 80.
We now have: 80 + 30 = 110 (8 + 3 = 11 and add zero at the end)
Now subtract the amount that was added to the two original numbers to get our final answer: 110 - 10 = 100
We could also have done this example the same way we did the previous two:
Round 22 down to 20 and 78 up to 80
Now we have: 20 + 80 = 100
Adding three two-digit numbers
Adding three numbers is pretty much the same as adding two. The only difference is now you will be rounding up or down three numbers and subtracting the totalof three numbers from the intermediate answer. We will do three examples since as mentioned, it's not much more difficult than what we did before.
43 + 39 + 77 Round 77 up to 80, 39 up to 40 and 43 up to 50. We've rounded up a total of 11 (3 + 1 + 7).
We now have: 80 + 40 + 50 = 170 (8 + 4 +5 = 17 and add zero at the end)
Now subtract the 11 that we added at the beginning to get the final
answer: 170 - 11 = 159 (170 - 10 = 160 - 1 = 159)
Let's do this example again a slightly different way. We'll round 43 down to 40 instead and leave the
others the same.
We now have: 40 + 40 + 80 = 160 (4 + 4 + 8 = 16 and add zero at the end)
When we rounded up we added 1 to 39 and 3 to 77 for a total of 4 and we subtracted 3 from 40.
In other words, 1 + 3 - 3 = 1. This is the amount that must be subtracted to get our final
answer: 160 - 1 = 159
As you can see there is more than way to solve the same problem. I usually use two different methods
when I want to make sure that a calculation is correct.
25 + 81 + 94 This time we will round everything down. That means that at the end instead of subtracting from the
intermediate answer we'll be adding to it to get the final answer.
Round 25 down to 20, 81 down to 80 and 94 down to 90. So we rounded down a total of 10.
We now have: 20 + 80 + 90 = 190 (2 + 8 + 9 = 19 and add zero on the end)
Now we must add the total amount that we rounded down to get the final
answer: 190 + 10 = 200
Let's do this example again by rounding up 94 to 100 instead.
We now have: 20 + 80 + 100 = 200
Did you notice that the intermediate answer is also our final answer? This is because we rounded up
and down by the same amount. We rounded down a total of 6 (25 to 20 and 81 to 80) and round up by
the same amount (94 to 100) so there was nothing to add or subtract.
