Use Compensation to Add up to 3 Digits
πͺ 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.
Do this: Read the concept below, then try the quiz or activity.
Concept
If you see a number like 399, your brain should immediately start ringing an alarm: "PRETEND IT IS 400!"
π― 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: ___