Q4. A man rows a boat at a speed of 15 mph in still water. Find the speed of the river if it takes her 4 hours 30
minutes to row a boat to a place 30 miles away and return.
1) 5 mph
2) 10 mph
3) 12 mph
4) 20 mph

• 1) 5 mph
  
  Let the speed of the river=x mph, then
  Time taken row 30 miles upstream and 30 miles downstream = 30/(15-x) + 30/(15+x) = 9/2
  => 10/(15-x) + 10/(15+x) = 3/2
  => 2[10(15+x) + 10(15-x)] = 3(15-x)²
  => 300 + 20x + 300 - 20x = 675 -3x²
  => x² = 25 or x=5
• 7 years agoHelpfull: Yes(8) No(0)
• 3). 12 mph
• 7 years agoHelpfull: Yes(2) No(2)
• Let the speed of the river=x mph, then
  Time taken row 30 miles upstream and 30 miles downstream = 30/(15-x) + 30/(15+x) = 9/2
  => 10/(15-x) + 10/(15+x) = 3/2
  => 2[10(15+x) + 10(15-x)] = 3(15-x)²
  => 300 + 20x + 300 - 20x = 675 -3x²
  => x² = 25 or x=5
• 7 years agoHelpfull: Yes(2) No(2)