In an 800 m race, A gives B a start of 80 m and still beats him by 20 seconds. If A runs at 8 m/s, what is the speed of B?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 6 m/s
\(\text{Time of A} = 800 \div 8 = 100\) s
With the start, B runs only \(800 - 80 = 720\) m, and he finishes 20 seconds after A:
\(\text{Time of B} = 100 + 20 = 120\) s
\(\text{Speed of B} = 720 \div 120 = 6\) m/s
Why the other options are incorrect
Option (1) 5 m/s — Incorrect. B would take \(720 \div 5 = 144\) s and lose by 44 seconds.
Option (3) 6.4 m/s — Incorrect. This uses the full 800 m for B: \(800 \div 125\). With the start B runs 720 m, taking 112.5 s, and loses by only 12.5 seconds.
Option (4) 7.2 m/s — Incorrect. B would take \(720 \div 7.2 = 100\) s, so the race would be a dead heat.