Because I am not so clever, I’d do this as a set of simultaneous equations
w = number of white roses
d = number of daisies
f = number of flowers
[MATH]50 + w + d +15 + 30 = f \implies f = d + w + 95.[/MATH]
[MATH]\dfrac{w}{f} = \dfrac{5}{33} \implies w = \dfrac{5f}{33}.[/MATH]
[MATH]\dfrac{d}{f} = \dfrac{3}{11} \implies d = \dfrac{3f}{11} = \dfrac{9f}{33}.[/MATH]
You are allowed to remember first year algebra in more advanced course. And once you know f, the rest of the problem requires second year arithmetic. There was a reason you took those courses.