The formula that you want to use to convert this one over is
Markup on Cost = (Markup on Selling Price) / (1 - Markup on Selling Price)
For this scenario that means taking
0.55 / (1 - 0.55) = 0.55 / 0.45 = 1.2222, meaning that the rate of markup on the cost of tomatoes is approximately 122.22%