Olympiad Maths Prep

Track / Stage 5 / 378 of 400 #978 of 2000

Problem 978

AIME late
Combinatorics Difficulty 5.9 Find the answer

14. (15 points) The distance between location A and location B is 360 kilometers. A truck loaded with 6 boxes of medicine is driving from location A to location B, while at the same time, a motorcycle starts from location B and heads towards the truck. The truck's speed is 40 kilometers/hour, and the motorcycle's speed is 80 kilometers/hour. When the motorcycle meets the truck, it unloads 2 boxes of medicine and returns to location B. It then immediately turns around... Each time the motorcycle meets the truck, it unloads 2 boxes of medicine and returns to location B. How long will it take to transport all 6 boxes of medicine to location B? How far will the motorcycle have traveled in total by then? (Neglect the time for loading and unloading the medicine.)

Official solution

【Solution】Solution: The time taken for the first meeting is: 360÷(80+40)=3360 \div(80+40)=3 (hours), at this point the motorcycle has traveled 80×3=24080 \times 3=240 (km);

Second meeting: After the meeting, the motorcycle returns to point B, traveling another 240 km, taking 3 hours.
During this time, the car travels another 120 km, leaving 240120=120240-120=120 (km) to point B,
The motorcycle turns around from point B and heads towards the car, the time taken for the second meeting is 120÷(40+80)=1120 \div(40+80)=1 (hour);
Third meeting: The motorcycle travels 80 km, after loading the medicine, the motorcycle reaches point B and travels another 80 km, the car travels 40 km,
Leaving 8040=4080-40=40 (km) to point B, taking 1 hour;
The third meeting takes 40÷(80+40)=1340 \div(80+40)=\frac{1}{3} (hour), at the meeting, the motorcycle travels 80×13=80380 \times \frac{1}{3}=\frac{80}{3} (km), and then travels another 803\frac{80}{3} km, taking 803÷80=13\frac{80}{3} \div 80=\frac{1}{3} (hour).
Therefore, the motorcycle travels a total of: 240×2+80×2+803×2=69313240 \times 2+80 \times 2+\frac{80}{3} \times 2=693 \frac{1}{3} (km); the total time taken is: 3×2+1×2+13×2=8233 \times 2+1 \times 2+\frac{1}{3} \times 2=8 \frac{2}{3} (hours).

Answer: To transport all 6 boxes of medicine to point B, it takes at least 8238 \frac{2}{3} hours, during which the motorcycle travels a total of 69313693 \frac{1}{3} km.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.