Operations & Word Problems

Depth · one concept from first principles · step 2 of 4

Find the unit amount

A car can travel 28.14 km28.14\ \text{km} on 2.1 L2.1\ \text{L} of petrol. If petrol costs 14301430 won per liter, how much does the petrol cost for a 335 km335\ \text{km} trip in this car? (The car covers the same distance on every liter.)

RatiosNumber system
Your answer
How to solve
Strategy Analyze the Units — Distance and money are not directly linked -- petrol sits between them. So the chain runs distance, then liters, then won, and the middle link is the distance one liter carries.
1STEP 1

Find the distance on one liter

Divide the distance by the fuel it took.

28.14 ÷ 2.1 = 13.4
2STEP 2

Find the fuel the trip needs

Divide the trip's distance by what one liter carries.

335 ÷ 13.4 = 25
3STEP 3

Work out the cost

Multiply the litres by the price of one litre.

1430 × 25 = 35750
Answer
35,750 won
Check the fuel: 13.4 × 25 = 335 km, exactly the trip, and 25 L at 1430 won is 35,750 won.
Takeaway

When two quantities are not directly linked, find the one in the middle and go through it.

  • Find the distance on one liter
  • Find the fuel the trip needs
  • Work out the cost
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