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?

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