To solve this problem you must apply the proccedure shown below:
1- The problem says that you are swimming each 25 yard length 3 seconds faster than your personal best and you have to calculate the change in time (which you can call ) of your personal best after 200 yards.
2- To calculate this, you must multiply 200 yards by 3 seconds and divide by 25 yards, as following:
Therefore, the answer is: 24 seconds.