The number of lights switched on in a room and the room's brightness.
i think it would be D i hope i helped
D + 3r = 15
d = r + 3
r + 3 + 3r = 15
4r + 3 = 15
4r = 15 - 3
4r = 12
r = 12/4
r = 3 ....he bought 3 roses, at $ 3 per rose = $ 9 <==
d = r + 3
d = 3 + 3
d = 6....he bought 6 daisies, at $ 1 per daisy = $ 6