Câu trả lời:

x/3=y/4 -> y=4x/3 (1)
y/5=z/6 -> y=5z/6 (2)

(1)+(2) -> x=5z/8 thay vào M=\(\dfrac{2.\dfrac{5z}{8}+3.\dfrac{5z}{6}+4z}{3.\dfrac{5z}{8}+4.\dfrac{5z}{6}+5z}\)=\(\dfrac{186}{245}\)