Answer
$R+L+I=120$
and $ 2.5 R+4L+2I=300$
and $R=2(L+I)$
Work Step by Step
We are given that there are $120$ flowers.
Let us suppose that $R$ is the number of roses, $L$ is the number of lilies and $I$ is the number of Irises.
Therefore, $R+L+I=120$
Further, the total price of the flowers is $ \$ 300$.
Thus, $ 2.5 R+4L+2I=300$
Next, $R=2(L+I)$
