For the first question, the answer is A. You take 90 and multiply it by 3, then you get the time that it takes for one worker to mow the large park.
For the second question, you take 50 = k/0.25. To find k, you would want to multiply both sides with 0.25 to cancel the 0.25 on k's side. 50 times 0.25 is 12.5. I think C is your answer although I'm not really confident.