Let w be the number of weeks. He withdraws $25 per week, so in w weeks, he withdraws 25w. He starts with $500, so after w weeks he has 500 - 25w. He wants this amount to be at least $200. "At least" means "greater than or equal to."
500 - 25w >= 200
-25w >= -300
w <= 12
Answer: Keith can withdraw $25 per week for at most 12 weeks.