You want to borrow three rock CDs from your friend. She loves math puzzles and she always makes you solve one before you can borrow her stuff. Here's the puzzle: Before you borrow three CDs, she will have 39 CDs. She will have half as many country CDs as rock CDs, and one-fourth as many soundtracks as country CDs. How many of each type of CD does she have after you borrow three rock CDs?

Rock CD's=21

Country CD's=12

Soundtrack CD's=3

Step-by-step explanation:

let

r= rock CD's

c = country CD's

s = soundtrack CD's

Total CD's=r + c + s =39

2c=r

c/4=s

2c+c+c/4=39

8c+4c+c=156

13c=156

c=12

Substitute c=12 into 2c=r

2c=r

3(12)=r

24=r

r=24

Substitute c=12 into c/4=s

c/4=s

12/4=s

3=s

s=3

Therefore, she has

c=12

s=3

r=24

before she borrowed you 3 rock CD's

Sha has

r=21

s=3

c=12

after you borrowed 3 rock CD's

687.09 dollars       happy now

step-by-step explanation: 6870

1857 times .63 equals 1169.91 subtract that from 1857 you get 687.09

a 4/6 = 9/12   not proportional

step-by-step explanation:

to find which are not proportional, we can use cross products

if it is true, they are proportional.   if it is not true, they are not proportional.

a 4/6 = 9/12

4* 12 = 6*9

48 = 54

not this is not proportional  

b 2/3 = 12/18  

2* 18 = 3 * 12

36 = 36

this is proportional

c 5/4 = 10/8  

5*8 = 4*10

40 = 40

this is proportional

d 7/5 = 35/25

7*25 = 5 * 35

175 = 175

is proportional