Use Compensation to Add on a Number Line
š Look out, it's a trick number! If a number ends in 8 or 9, it's secretly pretending to be the next 10. Let's expose its secret to add faster.
Concept
Adding numbers like 29 or 38 can be annoying. We have to make so many 1-hops on the number line! Let's use a shortcut. šÆ **The "Jump Too Far and Hop Back" Strategy:** **Example: 45 + 19 = ?** 19 is annoying. But 19 is incredibly close to a perfect 20! So... just pretend it IS 20! 1. **Jump Too Far:** Start at 45. Add a giant, easy 20. (45 + 20 = 65). 2. **Wait, we added too much!** We were only supposed to add 19. We added 1 too many! 3. **Hop Back to Compensate:** Take 1 tiny hop BACKWARDS to fix the mistake. (65 - 1 = 64). So, **45 + 19 = 64**. Adding 20 and taking away 1 is much, much faster than adding 19!
Try it
Jump too far, then hop back! **Instructions:** Pretend the annoying number is a perfect 10, jump forward, then hop backward to fix it! **Problem 1: 52 + 29 = ?** * Pretend 29 is 30! * Jump +30: 52 -> 82. * Wait! Hop backward -1: 82 -> ___ * Answer: ___ **Problem 2: 34 + 38 = ?** * Pretend 38 is 40! * Jump +40: 34 -> ___ * Wait! 38 is TWO less than 40. You have to hop backward -2! * Answer: ___ **Problem 3: 47 + 19 = ?** * Answer: ___ **Problem 4: 65 + 28 = ?** * (Jump 30, hop back 2!) * Answer: ___