goi do dai quang duong AB la x (km) (x>0)
thoi gian xe may di tu A den B la x/40(h)
thoi gian xe may di tu b ve a la x/30(h)
doi 45' =3/4 (h)
theo bai ra ta co pt
x/40+3/4=x/30
<=>3x/120+90/120=4x/120
=>3x+90=4x
<=>3x-4x=-90
<=>-x=-90
<=>x=90(TMDK)
vay quang duong AB dai 90 km