设柏树为70,松树为80,杨树为49
x = 7/8 y
z = 7/10x
x + y + z = 240
7/8 y + y + 7/10(7/8y) = 240
0.875 y + y + 0.6125 = 240
y = 240 / 2.4875 = 96 (大约值,非整数)
x = 84
x = 60 (这是以240 - 96 - 84 求的)