Multiple choice

The normal speed of a boat in still water is 4 times the speed of the river. The boat goes upstream and takes 2 hrs to reach from Q to P. By what percent should the boat increase/decrease its speed of it is required to reach P from Q in exactly 1 hr. 30 minutes? एक नाव की स्थिर जल में सामान्य चाल नदी की चाल का चार गुना है। नाव ऊर्ध्वप्रवाह जाती है और Q से P तक पहुँचने में 2 घण्टे लेती है। नाव P से Q तक नियत 1 घण्टे 30 मिनट में पहुँचने के कितने प्रतिशत से अपनी चाल में वृद्धि/कमी करनी होगी?

  1. +20%

  2. +25%

  3. –20%

  4. – 25%
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Let river speed = r km/hr. Then boat speed in still water = 4r km/hr. Upstream speed = boat speed - river speed = 4r - r = 3r km/hr. Time from Q to P = 2 hours, so distance = 2 × 3r = 6r km. Required time = 1 hr 30 min = 1.5 hours. Required speed = 6r ÷ 1.5 = 4r km/hr. Percentage change = (4r - 3r)/3r × 100 = 33.33%, but the answer is marked as -20%. The problem may have different interpretation.

Keep practicing — related questions