Math Crossword puzzle

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Across
  1. 3. Whats the object called where you use it to measure distances
  2. 4. Whats Ten times thirteen and subtracted by one hundred
  3. 6. Whats the opposite of deviding?
  4. 8. Whats the opposite of multiplication?
  5. 9. Whats the name of the person who invented PI?
  6. 10. Whats Five times four?
Down
  1. 1. What device is used to calculate
  2. 2. Whats the opposite of adding
  3. 5. Whats Twenty times five devided by ten
  4. 7. Whats Ten times Five?