Question

Walter Lewin plans to drive $900$ miles to Lonavla driving an average of $50$ miles per hour. How many miles per hour faster must he drive to reduce his total driving time by $3$ hours?

• A. 5
• B. 8
• C. 10
• D. 15
• E. 18

Answer 1

The correct option is C 10

Distance Lewin plans to drive $=900$ miles

His speed $=50$ miles/hr

Time taken by him to travel this distance $=$ $\frac{distance}{speed}=\frac{900}{50}=18$ hours

If he reduce his total driving time by $3$ hours, then time taken by him $=18-3=15$ hours

$\therefore$ his speed $=$ $\frac{distance}{time}=\frac{900}{15}=60$ miles\hr

Then he reduce his speed by $=60-50=10$ miles\hr