(2) from v=st:
t′= SV′= 30m 0.8m/s = 37.5s；
(3) The volume of gasoline consumed in 30s is v =10ml/10s× 30s = 30ml,
From ρ=mV:
The quality of gasoline consumed is m = ρ V = 0.8×103kg/m3× 30×10-6m3 = 24×10-3kg = 24g.
Answer: (1) The speed of Xiaojiang must be at least 1.5m/s to pass safely;
(2) The total countdown time of the green light must be set to at least 37.5s before grandma can pass safely;
(3)30s consumes 24g. Gasoline.