CAT 2015 Quantitative Ability

In a rowing competition, first boat rows over the course at an average speed of 4 yards/second Second boat rows over the first half of the course at 1 the rate of $3\frac{1}{2}$ yards /second and over the remaining half at $4\frac{1}{2}$ yards/second, thereby reaching the winning post 15 seconds after the first boat. Find the time taken (in minutes) by the second boat to cover the entire course