Maths problem: how to do (c) and (d), thanks?

(c) Methods of buying 2 books using ≦ $50 are:
{16+16,16+20,16+28,16+34, 20+20, 20+28}

∴ no. of ways buying 2 books using ≦ $50
= C(6,2)+6*8+6*4+6*6+C(8,2)+8*4
= 15+48+24+36+28+32
= 183

(d) Methods of buying 1 book using ≦ $50 are:
{16, 20, 28, 34}
∴ no. of ways buying 1 book using ≦ $5
= 6*8*4*6
= 1,152‬

Method of buying 3 books using ≦ $50 is:
{16+16+16}
∴ no. of ways buying 1 book using ≦ $5
= C(6,3)
= 20

∴ no. of ways of buying any no. of books using ≦ $50
= 183+1152+20
= 1,355