I think you meant how many roses did he buy?
If not, then the easy answer is $3 each! :)
But assuming you meant how many roses did he buy:
We must find the value for the number of daisies bought, d, which satisfies the following equation:
$27 = $1(d) + $3(d - 3)
Using the very sophisticated guess and check method we find that the answer is:
d = 9
Therefore, the number of roses bought is 6.