Question

ken bought d dozen roses for r dollars represent the cost of one rose
A. r/d
B. 12d/r
C. 12r/d
D. r/12d

Answer 1

Suppose you had actual numbers here:
Kken bought 5 dozen roses for $72.00 dollars represent the cost of one rose

What steps would you take to find the cost of 1 rose to be $1.20 ?
Now repeat those same steps using the variables d and r.

Answer 2

you can always multiply by 1 without changing anything. So your conversion from roses to dollars goes like this:
1 rose * 1 dozen/12 roses * r dollars/d dozen = r/12d dollars/rose