To answer this question, we can translate this situation algebraically as follows:

Sadie already has $75 saved, and still needs to save what is earned each week at McDonald's ($25 per week). Then, solving for x, we have:
1. Subtract 75 from both sides of the equation:

2. Divide both sides of the equation by 25:

Therefore, Sadie has to work for 5 weeks to earn enough money to buy the sneakers.