a/3=b/7=c/8=k
=>a=3k
b=7k
c=8k
=>2•3k+5•7k-4•8k=108
=>6k+35k-32k=108
=>41k-32k=108
=>9k=108
=>k=108/9=12
=>a=3•12=36
b=7•12=84
c=8•12=96
a/3=b/7=c/8
a=3k
b=7k
c=8k
2a+5b-4c=108
2×3k+5×7k-4×8k=108
6k+35k-32k=108
41k-32k=108
9k=108
k=108:9
k=12
a=3×12
a=36
b=7×12
b=84
c=8×12
c=96
