answer: about 35 roses
explanation: make 2 equations, one for price and one for number sold
let r be roses and t be tulips.
r + t = 2r
2t + 5r = 240
isolate for t in the second equation and substitute it into the first:
2t = 240 - 5r
t = 120 - 5/2r
r + (120 - 5/2r) = 2r
120 = 2r + 5/2r - r
120= 7/2r
240/7 = r
35 = r (approximately)