Fuel per km, then fuel for 15 km

A car consumes 0.5 L of fuel to travel 6 km. At this rate, how many liters of fuel are needed to travel 15 km?

Step-by-step

1. 0.5 ÷ 6 = 0.0833
   Find the unit price first (reduce to one)
2. 0.0833 × 15 = 1.25
   Unit price × quantity

Why it works

The unitary method involves first calculating "how much corresponds to one unit," then multiplying by the required number of units. Fuel consumption is directly proportional to distance, so we first find the fuel used per km and multiply by 15 km. The unit value is often not an integer; keeping it as a fraction or exact decimal avoids rounding errors.

Common mistakes

• Reversing the order of division and multiplication by dividing 6 by 0.5.
• Rounding before multiplying, causing the last digit to differ from the standard answer by one place.
• Confusing "fuel per km" with "km per liter," writing the reciprocal instead.

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