Is there a proportional relationship between time and the amount of water left in the tank?

Yes

Step-by-step explanation:

y is the litters of water in the tank

x is the time in minutes

m is the change in litters

b is the initial amount of water

so the formula is y = mx + b

whereas m is -14 and b is 770

y = -14x + 770 or y = 770 - 14x

since the airplane is moving with constant speed then the distance travelled can be calculated by x=u*t

in km/h would be:

x=640*3.5=2.240 km

in m/s would be:

u=640 km/h= 640.000 m/h =640.000/ 3600 m/s =177.8 m/s

t= 3.5 h= 3.5 * 3600 s=12.600 s

then x=u*t= 177.8*12.600= 2.240.280 m