Speed = distance ÷ time, and the other two follow by rearrangement: distance = speed × time, time = distance ÷ speed. Most misses multiply where they should divide (or vice versa) — often after a units mismatch.
Three quantities, two operations, time pressure: the triangle collapses. Add mixed units (minutes with mph) and even a correct rearrangement produces a wrong number.
Use the triangle: cover the quantity you want, what remains is the formula. Convert units FIRST (90 minutes = 1.5 hours), then compute. Sanity check with real-world feel: a car doesn't cover 400 miles in 90 minutes.
Travelling at 40 mph for 5 hours, how far do you travel?
Average speed = total distance ÷ total time. Never average the speeds themselves. The correct answer is 200 miles. Traps to avoid: 100 miles comes from the "sdt rearrange error" error; 45 miles comes from the "sdt rearrange error" error; 240 miles comes from the "sdt rearrange error" error; 5 miles comes from the "sdt rearrange error" error.
This trap appears in 10 of our questions, across: Speed, distance, time.
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