Understanding the Problem:Think of this as a "race against time" problem. The ship is filling with water, but the pumps are fighting back. We need to find how long the ship can stay afloat, then figure out how fast it needs to go to cover
180 km in that time.
Step 1: Find how fast water is coming INThe leak lets in
3 tons every
8 minutes.
To convert to tons per hour:
Rate = (3 tons / 8 minutes) × 60 minutes = 22.5 tons per hourStep 2: Find the NET rate of water accumulationWater coming IN:
22.5 tons/hour
Water being pumped OUT:
12 tons/hour
Net rate = 22.5 - 12 = 10.5 tons per hourEven with pumps working, the ship still gains
10.5 tons of water every hour.
Step 3: Find how long until the ship sinksThe ship sinks after taking in
90 tons of water.
Time = Total capacity ÷ Net rate = 90 ÷ 10.5Let's simplify: 90 ÷ 10.5 = 900 ÷ 105 =
60/7 hours (approximately 8.57 hours)
Step 4: Calculate the required speedThe ship needs to travel
180 km in
60/7 hours.
Speed = Distance ÷ Time = 180 ÷ (60/7) = 180 × (7/60) = 1260/60 = 21 km/hKey Principle:In problems with competing rates (filling vs emptying), always calculate the NET rate first. Make sure all units match (convert minutes to hours when needed).Answer: C (21 km/h)