The problem of mathematics

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Across
  1. 3. 410,674+420,6316
  2. 7. 963,258-369,820
  3. 9. 999,999-999,888
  4. 10. 987,654+147,963
  5. 13. 500000+40000+2000+300+30+2
  6. 14. 985,576-652,963
  7. 17. 741,258-740,259
  8. 18. 629,323 rounded to 1,000
  9. 20. 60000+7000
  10. 21. 663,125-214,668
Down
  1. 1. 623832+524,662
  2. 2. 123,458+789,123
  3. 4. 532,782+328,642
  4. 5. 412,952+610,687
  5. 6. 60000+2000+100+50+3
  6. 8. 852,201-589,147
  7. 11. 369,982+652,182
  8. 12. 532,632 rounded to 10,000
  9. 15. 988,987-789,663
  10. 16. 639,343 rounded to 10
  11. 19. 305,621+541,756