Subtracting 9
š 9 is almost 10! Because 9 is just one step away from 10, it has the absolute coolest math shortcut of them all.
Concept
Get ready for the fastest subtraction trick in the world! šÆ **The "Magic Plus 1" Trick:** Because 9 is only 1 step away from 10, subtracting 9 from a teen number is magical. **Example: 15 - 9 = ?** All you have to do is take the two digits in the number 15 and add them together! What is 1 + 5? It's 6. So, **15 - 9 = 6**! Let's try another: **13 - 9 = ?** Add the digits: 1 + 3 = 4. So, **13 - 9 = 4**! Why does this work? Because instead of subtracting 10, you are subtracting 9. So you subtract a whole bundle, but you have to "give 1 back."
Try it
Let's try the Magic Plus 1 trick! **Instructions:** Add the digits of the teen number together to find the answer. **Problem 1: 14 - 9 = ?** * Add the digits: 1 + 4 = 5. * Answer: 14 - 9 = ___ **Problem 2: 17 - 9 = ?** * Add the digits: 1 + 7 = 8. * Answer: ___ **Problem 3: 12 - 9 = ?** * Add the digits: ___ + ___ = ___ * Answer: ___ **Problem 4: 18 - 9 = ?** * Answer: ___