∵ a/3 = b/4 = c/5
∴设 a=3x,b=4x,c=5x
a-2b+3c = 5
3x - 8x+15x = 5
10x = 5
x = 1/2
2a+b - 4c
= 6x + 4x - 20x
= - 10x
= - 5