USE MENTAL MATH TO ADD OR SUBSTRACT: USING COMPATIBLE NUMBERS
1. 80+79+20
2. 308+95+5
3. 480+255+20
4.365+289
5. 523-198
6. 537+180
7. 325+239+75
8. 87+38+62
9. 4500-280
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1. 80+79+20=100+20+59=120+59=179
2. 308+95+5=300+100+8=400+8=408
3. 480+255+20=480+20+255=500+255=755
4. 523-198=500-200=300 AND 23+2=25 THUS 300+25=325
The first three are right.
You didn't do # 4.
Number 5 is correct.
To use mental math with compatible numbers, we look for numbers that are close to each other and easy to work with. Here's how you can approach each problem:
1. 80 + 79 + 20:
Group the compatible numbers: 80 + 20 + 79
Add the compatible numbers: 80 + 20 = 100
Add the remaining number: 100 + 79 = 179
2. 308 + 95 + 5:
Group the compatible numbers: 308 + 5 + 95
Add the compatible numbers: 308 + 5 = 313
Add the remaining number: 313 + 95 = 408
3. 480 + 255 + 20:
Group the compatible numbers: 480 + 20 + 255
Add the compatible numbers: 480 + 20 = 500
Add the remaining number: 500 + 255 = 755
4. 365 + 289:
These numbers are not as compatible, but we can still use mental math to add them:
Round 365 to 360 and round 289 to 290.
360 + 290 = 650
5. 523 - 198:
Round 523 to 520 and round 198 to 200.
520 - 200 = 320
6. 537 + 180:
These numbers don't have any compatible numbers, but we can still do mental math:
537 + 180 = 717
7. 325 + 239 + 75:
Group the compatible numbers: 325 + 75 + 239
Add the compatible numbers: 325 + 75 = 400
Add the remaining number: 400 + 239 = 639
8. 87 + 38 + 62:
These numbers are all compatible, so we can add them easily:
87 + 38 + 62 = 187
9. 4500 - 280:
Round 4500 to 4500 and round 280 to 300.
4500 - 300 = 4200
By using compatible numbers and mental math techniques, you can quickly add or subtract numbers without the need for a calculator.