1) Let the function be q = at+b.

60 = b；

20 = 8a+b； A=-5, b=60, so the function is q = 60-5t;

2)Q=60-5t= 15, and t=9, then 40x9 = 360km.

3) If the meaning of the problem is 60-5t=0, and t= 12 hours, and the oil quantity is zero, then

You must return after running for 6 hours, that is, 6*40=240 kilometers.

Thank you. Have a good time. Remember to adopt it if it helps.

I wish you progress in your study.