Consider the essence of the problem: each client will only buy ice cream if the price is less than or equal to the amount they are willing to spend. Therefore, for any given price, only the clients with a budget equal to or greater than this price will make a purchase.
Now, ponder on this: if you sort the clients based on the amount they are willing to spend and then iterate through these sorted values, you can determine how many clients will buy the ice cream at each of those price points. This way, you create intervals of possible prices.
Reflect on the following: for each potential price that corresponds to a client’s budget, calculate the profit you would make if you set the ice cream’s price at that value. Keep in mind the cost of production k. You seek the maximum profit, which is the maximum of these calculated profits.
The hint I shall bestow upon you is to consider the sorted budgets of the clients as the potential prices for the ice cream and to calculate the profit for each of these prices. The profit at each price is the number of clients who would buy at that price multiplied by the profit margin (price - k). The price that yields the highest profit is the solution to your quest.