Question

A boat traveled 210 miles downstream and back. The trip downstream took 10 hours. The trip back took 70 hours. What is the speed of the boat in still water?

• A. 9 mph
• B. 13 mph
• C. 8 mph
• D. 12 mph

Answer 1

The correct option is D 12 mph

Let speed of Boat be 'b' and speed of Current be 'c'. Distance given is 210 miles.
Therefore speed for downstream = $\frac{210}{10}=21mph$.
Speed for upstream is $\frac{210}{70}=3mph$. Now we can form the two equations as :
i) b+c = 21 ......(i) For downstream
ii) b-c = 3.........(ii) For Upstream [Note : speed of boat must be more than speed of current, otherwise the boat will go backwards ]
Solving by substitution method :
From equation (ii) c = b - 3 Substitute this value of c in equation (i), we get
b + b - 3 = 21
Therefore b = 12. Now substitute the value of b in any of the equations above (i) or (ii) and get c= 9
b= 12 and c = 9.
The correct answer is 12 mph. Option D