Answer:
13. Twenty seven minus two times a number x.
14. Three times two plus four.
- I don't know if this x is a variable or a multiplication sign, but I'm assuming it's a multiplication sign.
15. Fifteen times a number y.
16. Three times two minus two times a number x.
17. Fifty four
18. Six times a number x minus two.
19. Two times a number x plus sixteen.
20. Four divided by a number n.
These could also be phrased using vocabulary like "the sum of two times a number x and sixteen." If you want this, please comment and I'll add it!