Question

A boat, while going downstream in a river covered a distance of $50$ miles at an average speed of $60$ mph. While returning, because of the water resistance, it took $1$ hour $15$ minutes to cover the same distance. What was the average speed during the whole journey?

• A. $40$ mph
• B. $48$ mph
• C. $50$ mph
• D. $55$ mph

Answer 1

The correct option is B $48$ mph

Time taken to cover $50$ miles downstream $=\frac{50}{60}=\frac{5}{6}$ hrs
Time taken to cover $50$ miles upstream $=1$ hr $15$ minutes $=\frac{5}{4}$ hrs
Total time taken to cover $100$ miles $=(\frac{5}{6}+\frac{5}{4})=\frac{25}{12}$ hrs
$\therefore$ Average speed $=\frac{100}{\left(\frac{25}{12}\right)}=\frac{100\times 12}{25}=48$ mph