Convert 15 US gallons to litres

Question

Solve in your head. What is 15 US gallons in litres? Aim: under 15 seconds.

Accepted Answer

One US gallon is about 3.785 litres. Rather than multiply by 3.785, use 4 - 0.215: 15 	imes 4 = 60, \qquad 15 	imes 0.215 \approx 3.2. 60 - 3.2 \approx 56.8 	ext{ litres.} So about 57 litres (exact 56.78). Takeaway. Turn an ugly conversion factor into "a round number minus a small correction." 	imes 3.785 becomes 	imes 4 (trivial) minus a \approx 5\% trim. The same move handles 	imes 0.62 as 	imes 	frac23 minus a hair, and 	imes 1.61 as 	imes 1.6 plus a hair.

Convert 15 US gallons to litres

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