5 noble fir and 3 douglas fir = $ 420
12 noble fir and 9 douglas fir = $ 1080
Let noble fir be n, and douglas fir be d.
5n + 3d = 420 ..............(i)
12n + 9d = 1080 ...............(ii)
Multiply equation (i) by 3.
3*(5n + 3d) = 3*(420)
15n + 9d = 1260 .............(iii)
Equation (ii) minus (iii)
(12n + 9d) - (15n + 9d) = 1080 - 1260
12n - 15n + 9d - 9d = -180
-3n = -180
n = -180/-3 = 60
Substitute the value of n in (i) 5n + 3d = 420
5*(60) + 3d = 420
300 + 3d = 420
3d = 420 - 300 = 120
3d = 120
d = 120/3 = 40
Therefore Noble fir tree cost $60 while Douglas fir tree cost $40
