Answer
See below.
Work Step by Step
Let $x$ be the number of roses, then the number of canatations is $20-x$, thus the total cost is: $3x+1.5\cdot(20-x)=39\\3x+30-1.5x=39\\1.5x=9\\x=6$
Thus we have $6$ roses and $20-6=14$ carnatations.
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