Question

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?

Answer 1

Rock CD's=21

Country CD's=12

Soundtrack CD's=3

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