Mathematical Terms Puzzle

Across

2. Number after eighteen and before twenty

4. Three times as much

7. Branch of mathematics with symbols

8. Complete and without reservation

13. Three times over in old fashioned terms

14. Number following nine hundred ninety nine

15. Logical and reasonable thinking process

Down

1. First letter of the Greek alphabet

3. Greek letter following delta in alphabet

5. Symbol representing a number value

6. Position after sixth in sequence

9. Position after seventh in sequence

10. Zero or nothing at all

11. One fourth part of something

12. Quantity with magnitude but no direction