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There are a couple of ways you can do the addition in your head.

I would probably just break it down as I would do on paper.

155

+83

---

So, 15+8 = 23

5+3 = 8

So your answer is 238

Now, when I'm doing basic long addition in my head, I always work from left to right... knowing that one has to be careful about carrying digits. But, it is easier to think from left to right than to think the other way around.

Sometimes one can make the equation a little more complicated and it will actually become easier to solve.

For example, you could think of the equation as:

155 + (83 + 17) - 17

In this case you end up with:

155 + 100 - 17 = 255 - 17 = 238.

Often similar types of manipulating the numbers into something easier helps when doing multiplication in one's head.

Another example of "changing the equation" was calculating time when I was traveling in Italy, I had to convert from European time to US Pacific Standard time quite frequently which was 9 hours earlier.

So, rather than subtracting 9 hours, I would always add 3 hours and subtract 12 hrs. So, if it was 4 in the afternoon, add 3 hrs to get 7, then subtract 12 and get 7 AM.

Q: How can you add 155 plus 83 using mental math?

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The answer is 155. Simply add up all of the numbers arithmetically like you would for two plus two. To make sure the answer is correct, check your work using a calculator.

155 * 97 = 155 * (100 - 3) = 155*100 - 155*3 = 155*100 - (150 + 5)*3 = 155*100 - 150*3 - 5*3 = 15500 - 450 - 15 = 15035