Write the known conversion as a ratio, set it equal to your problem, and cross multiply. This works for any conversion.
Convert 7 feet to inches (1 foot = 12 inches)
Set up: 1 ft / 12 in = 7 ft / x in
Cross multiply: 1 × x = 12 × 7
x = 84 inches
Multi-Step Conversions
Convert 2.5 hours to seconds
Hours → minutes: 2.5 × 60 = 150 minutes
Minutes → seconds: 150 × 60 = 9,000 seconds
Convert 50 km/h to m/s
km to m: 50 × 1,000 = 50,000 m per hour
hours to seconds: 50,000 ÷ 3,600 ≈ 13.9 m/s
When to Multiply vs Divide
Converting to a smaller unit → the number gets bigger (multiply).
Converting to a larger unit → the number gets smaller (divide).
5 feet → inches: going smaller, so the number increases (60). 60 inches → feet: going bigger, so the number decreases (5).
💡 Sanity check: Does your answer make sense? 5 feet should be many inches (bigger number), not a fraction. If the number went the wrong direction, flip your conversion ratio.
🎯 Practice Problems
1. Convert 4.5 pounds to ounces. (1 lb = 16 oz)
A) 3.56 oz
B) 20.5 oz
C) 72 oz
D) 0.28 oz
2. Convert 3,000 meters to kilometers. (1 km = 1,000 m)
A) 0.3 km
B) 3 km
C) 30 km
D) 3,000,000 km
3. Convert 2 days to minutes.
A) 120 min
B) 1,440 min
C) 2,880 min
D) 172,800 min
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