Jan 24, 2019 15:35:29

When we say numbers, we say them left-to-right but when we subtract we are taught to subtract right-to-left 🤯

Take a second and think about it: left-to-right.

This method is great when you are trying to do quick mental math calculations. This is how you do subtract left-to-right:

34
- 23
?

• 3-2 = 1
• 4-3 = 1
• 34 - 23 = 11

Check if the second top number is greater or less than the second bottom number (4 > 3?). If it is greater then just subtract the first top number(3) with the second bottom number(2). If it is less than then subtract one from the first top number and then subtract it with the first bottom number.

53
- 24
?

• 4 - 2 = 2
• 13 - 4 = 9
• 53 - 14 = 29

Since the second top number (3) is less than the second bottom number (4), we subtract one from the first top number (5 -1) before subtracting it with the first bottom number (2).

Then add 10 to the second top number (3 +10) and subtract the second bottom number (4) to get the final answer.

Here is another example:

320
- 245
?

• (3-1) - 2 = 0
• (12-1) - 4 = 7
• 10 - 5 = 5
• 320 - 245 = 75

Practice a couple more to get familiar. It's hard to let go of habits that are ingrained in our brains. If it was hard to follow through but like the idea of it, watch some videos to learn about how others think through when they are subtracting left-to-right. This is the method that works the best for me.

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