Use Compensation to Add up to 3 Digits
šŖ The Grand Illusion! We can pretend a scary 3-digit number is just a perfect, round Hundred! But don't forget to fix the illusion at the end.
Concept
If you see a number like **399**, your brain should immediately start ringing an alarm: "PRETEND IT IS 400!" šÆ **The "Steal and Fix" Strategy:** **Example: 245 + 198 = ?** Adding 198 is incredibly annoying and requires double carrying. 1. **The Trick:** 198 is secretly pretending to be 200! So just add 200! 2. **The Easy Math:** 245 + 200 = 445. 3. **The Fix:** Wait! I added 200. I was only supposed to add 198. How far off was I? I added TWO TOO MANY. 4. **Give it back:** Subtract 2 to fix the mistake. (445 - 2 = 443). 5. **Answer:** **443**! You just added 3 digit numbers in your head using subtraction!
Try it
Pretend it's a hundred! **Instructions:** Turn the scary number into a perfect hundred, add it, then SUBTRACT the extra! **Problem 1: 312 + 199 = ?** * Pretend 199 is 200! * 312 + 200 = ___ * Wait! I added ONE too many. Subtract 1! * Answer: ___ **Problem 2: 540 + 298 = ?** * Pretend 298 is 300! * 540 + 300 = ___ * Wait! I added TWO too many. Subtract 2! * Answer: ___ **Problem 3: 175 + 399 = ?** * Answer: ___ **Problem 4: 620 + 98 = ?** * Pretend 98 is 100! * Answer: ___