jack = 3 presents every 15 minutes
luke = 4 presents every 20 minutes
target = 20 presents wrapped per person
y = kx ; where k is the constant rate, x is the number of presents wrapped, y is the total number of minutes spent wrapping x number of presents.
we need to compute for the unit rate of each individual.
Jack: 15 minutes / 3 presents = 5 minutes per present
Luke: 20 minutes / 5 presents = 4 minutes per present
Jack:
y = 5 minutes per present * 20 presents = 100 minutes
Luke:
y = 4 minutes per present * 20 presents = 80 minutes