Write the phrase as a mathematical expression:
15. thrice the difference of y and two
16. the difference of 10 and the quotient of s and 4
17. the sum of 6 and product of 9 and z
18. four more than the quotient of 5 and b
19. the product of 2 and the difference of h and 7
20. the product of the difference of g and 1 and 3
