Use Compensation to Subtract on a Number Line
š¦ Outfox the problem! If you need to subtract an annoying number like 39, it's much faster to subtract a massive 40 and give exactly 1 back to balance the scale.
Concept
Compensation subtraction is tricky, but it's the coolest mental math secret when numbers end in 8 or 9. šÆ **The "Steal Too Much and Give Back" Strategy:** **Example: 75 - 29 = ?** Subtracting 29 is annoying. Let's pretend it is 30! 1. **Steal Too Much:** Start at 75 on the number line. Take a giant -30 leap backwards! * 75 - 30 = 45. 2. **Wait, we stole too much!** We were only supposed to subtract 29, but we subtracted 30. We took 1 too many! 3. **Give 1 Back (Hop Forward):** You hopped backward too far. Take 1 hop FORWARD on the number line to fix it. * 45 + 1 = 46. (We are HOPPING FORWARD because we gave 1 back!). So, **75 - 29 = 46**. Subtracting 30 (+1) is so much faster than writing down massive subtraction algorithms.
Try it
Steal too much, give 1 back! **Instructions:** Jump backwards with a perfect "10", then hop exactly 1 space forward to fix the mistake! **Problem 1: 62 - 19 = ?** * Pretend 19 is 20! * Jump backwards 20: 62 - 20 = ___ * Hop forward 1 to give it back: 42 + 1 = ___ * Answer: ___ **Problem 2: 84 - 39 = ?** * Pretend 39 is 40! * Jump backwards 40: 84 - 40 = ___ * Hop forward 1! ___ + 1 = ___ * Answer: ___ **Problem 3: 56 - 29 = ?** * Answer: ___ **Problem 4: 91 - 58 = ?** * (Hint: Pretend it's 60. Then hop forward 2!) * Answer: ___