HELP. Physics homework?

Question

Sally travels by car from one city to another. She drives for 30.0min at 80.0 km/h, 12.0 min at 105 km/h, and 45.0 min at 40.0 km/h, and she spends 15.0 min eating lunch and buying gas.

a. Determine the average speed for the trip.

b. Determine the total distance traveled.

Answer 1

Distance = speed * time

1) She drives for 30 min i.e. 30/60 = 1/2 hr at 80 km/h. Distance covered = 1/2 * 80 = 40 km

2) She drives for 12 min i.e. 12/60 = 1/5 hr at 105 km/h. Distance covered = 1/5 * 105 = 21 km

3) She drives for 45 min i.e. 45/60 = 3/4 hr at 40 km/h. Distance covered = 3/4 * 40 = 30 km

4) She spends 15 min i.e. 15/60 = 1/4 hr eating lunch and buying gas. Distance travelled during this time = 0.

Total distance travelled = 40 + 21 + 30 + 0 = 91.0 km

Total time spent = 1/2 + 1/5 + 3/4 + 1/4 = 17/10 hr

Average speed = total distance/total time = 91/(17/10) = 910/17 km/hr = 53.5 km/h

Ans: average speed = 53.5 km/h

total distance = 91.0 km

Answer 2

A = 53.52 km/h

B = 91.0 km

A = speed = distance/time = 91 km/1.7 h = 53.52 km/h

B = total distance = d1 + d2 + d3 = 40 km + 21 km + 30 km = 91.0 km

Answer 3

d = v1t1 + v2t2 + v3t3

d = 0.5*80 + 0.2*105 + 0.75*40

d = 40 + 21 + 30

d = 91

v = d/t

v = 91 / (.5+.2+.75 + .25)

v = 91 / 1.7

v = 53.53

I can't think of an easy way to get the average speed without calculating the total distance first.