On a river, B is between A and C and is equidistant from A and C. A boat goes from A to B and bank in 5 hours 15 minutes and from A to C in 7 hours. How less will it take to go from C to A, if water flows from A to C.

River is flowing from A to C, that means A to C is downstream and C to A is upstream. 

Since, Time taken to go from A to C in downstream = 7 hours

Time taken to go from A to B in downstream = $$\frac{7}{2} = 3.5 hours$$

Since, time taken to go from A to B and come back to the same point = 5 hours 15 minutes = $$5 + \frac{15}{60} = 5.25 hours$$

Now, time taken to go from B to A = 5.25 - 3.5 = 1.75 hours

Hence, time taken to go from C to A = 1.75 * 2 = 3.5 hours = $$3\frac{1}{2} hours$$