Question

The average speed of a train running at a speed of 30 Km/h during the first 100 kilometres, at 40 Km/h during the second 100 kilometres and at 50 Km/h during the last 100 kilometres is nearly:

• A. 38.5 Km/h
• B. 38.3 Km/h
• C. 40.0 Km/h
• D. 39.2 Km/h
• E. None of these

Answer 1

The correct option is B 38.3 Km/h

Time taken to cover first 100 kilometres
$=\frac{100}{30}=3\frac{1}{3}h$
Time taken to cover second 100 kilometres
$=\frac{100}{40}=2\frac{1}{2}h$
Time take to cover last 100 kilometres
$=\frac{100}{50}=2h$
Total time taken $=3\frac{1}{2}+2\frac{1}{2}+2=\frac{10}{3}+\frac{5}{2}+2=\frac{47}{6}h$
Total distance covered = 300 Km
$\therefore$ Average speed $=\frac{300}{\frac{47}{6}}=\frac{300\times 6}{47}=38.3Km/h$