a) x/y=2/7
x=2k
y=7k
3x-4y/2x+5y=3×2k-4×7k/2×2k+5×7k=6k-28k/4k+35k= -22k/39k= -22/39
b) x/y=2/9
x=2k
y=9k
6x+2y/3y+2x=6×2k+2×9k/3×9k+2×2k=12k+18k/27k+4k=30k/31k=30/31
c) x/y=3/7
x=3k
y=7k
20x-2y/4x+5y=20×3k-2×7k/4×3k+5×7k=60k-14k/32k+35k=
=46k/67k=46/67
