The time needed at 2 meters per second for both rounds would be 100/2 s or 50 seconds. For the first lane he already used 50/1.5 -> 33.33 seconds, so he would need to cover the other 50 meters in exactly 16.67. 50m divided by the 16.67 seconds he has left means he has to swim at a speed of 3 meters per second.