Nancy can clean a garden in 5 hours, and assuming she works at a constant rate, that means in 2 hours she has cleaned 40% of the garden. 2/5 = 0.4

For Nancy, N = 0.2 gardens cleaned per hour. 1 garden / 5 hours = 0.2 gardens/hour

With 0.4 of the garden cleaned, 0.6 remains. 1 - 0.4 - 0.6

(N + P) x 1 hour = 0.6 garden cleaned

P = 0.6 garden/hour - N = 0.6 - 0.2 = 0.4 garden/hour

Peter cleans 0.4, or 40% of a garden in an hour, so to clean 1 garden takes Peter 2.5 hours. 1/0.4 = 2.5

Peter would take 2.5 hours to clean the garden alone.